What you need to do is look at the specs of the tow vehicle and trailer and realistically decide where your loading will be relative to those. So, for the 171, max trailer weight is 3252 lbs. You will very likely be near that weight with a fully loaded trailer, especially if you plan to boon dock at all (carrying a full fresh water tank and dual batteries, gear, supplies, etc). If you never plan to camp without water and electrical hookups you can subtract about 300 lbs. 

Next, look at your driver's door sticker and find the MCGVWR (the max combined gross vehicle weight rating). That is the most that Subaru allows your total rig to weigh. Take your Suby's curb weight (3884 lbs I think), add the fully loaded trailer weight, and then add a realistic estimated weight of all the passengers and gear (grills, bikes, tents, coolers, folding chairs etc etc) you plan to carry in the vehicle. 

Compare those numbers, If you are over the MCGVWR don't do it, you will be overloaded, unsafe, and open yourself to significant liability if you have an accident. If you are under, then its a personal decision about how much safety factor you want. I personally want to be at least 10% under the MCGVWR all the time.

The other spec to check is hitch weight. What is the max hitch weight on the Subaru? To mitigate against sway, the trailer needs to be balanced so at the very minimum you have 10% of trailer weight on the hitch under all load conditions. 11-12% is preferable. So that is pretty much right at 350 lbs for a fully loaded 171. Ignore Forest River's dangerously low published hitch weight. 

Check the towing section of you manual to see if Subaru allows you to use a weight distribution hitch. That doesn't give you more hitch capacity but it does significantly improve ride and handling of the rig. 

Oh, btw, I think subaru doesn't recommend WDH, and has a low tongue weight, about 200 I believe. An Outback would be good for a small pop up though.

The Ascent weighs about 4500 lbs and has a 114 inch wheelbase. The Outback weighs 3900 lbs and has a 108 inch wheelbase. Those are big differences.  

The internet is great for finding folks who do all kinds of things.  But the real question is what does the manufacturer say, what will your trailer, hitch, and tow vehicle end up weighing when loaded (Hint: its always more than you think) and, assuming you are within specs, how close to the limits are you comfortable with? 

 Its all about weight. Go weigh the trailer, trailer tongue, tow vehicle and total combined weight as loaded for travel and compare those numbers to your specs. If you are under on all of them and you feel comfortable, fine. If you are over on any of them then you are not legal so please don't do it. 

Another aspect that hasn't been mentioned are the other important parts of the tow vehicle--the brakes and the transmission.

We initially towed our 173 with a Chevy Astro van. It had the power, but the transmission was not up to the task. After we replaced the transmission. we bought a used tow vehicle that will do the job.

Horsepower is only that-power, it doesn't always move you down the road, up the hills or stop you going down the hills.

You need a transmission cooler and serious brakes at a minimum. the rear axle ratio is also important to keep the rpm's down and the  transmission working long term.

Be careful-your tail will wag the dog easier than you know at this point in your towing career.

Vann

mjlrpod, actually the old VW's did great in the snow. The light front end, flat body pan and rear weight over the drive wheels allowed the old bugs to ride up over big drifts without getting stuck.  If a huge drift stopped them all you had to do was back up and hit it again till you pushed through.  I had one living in Cambridge during the blizzard of '78 and I was driving all over Boston in it 'till the police stopped me to tell me they had shut down the city and I had to go home and park it. 

Yep, the VW heater was ineffectual, I used to scrape the inside windshield too. It was close to your face so at least it was easy to reach.   I had  56, 59, 68, and 72 bugs. With studded snow tires I didn't have any problems in them with snow as long as I could see where I was going. Never tried the pond skimming, that does sound like fun. 

If you think the bugs were bad the buses were worse. I drove a 40HP 60 bus across on the trans Canada highway from Vancouver in January. Got as far as Winnipeg and gave up, headed south. That was an early Westphalia camper, my first RV. Wish I had that now, it would be worth a small fortune. Boy was it cold, and slow. 

But those things were hard to stop and easy to fix so for a poor student they were great. I recall clutch disks costing a whole six bucks and taking 30 minutes to change, start to finish. The good old days when cars with easy to fix and we were crazy.... 

I personally kinda think the whole unsafe at any speed thing was a bit over the top. Lots of cars were dangerous back then besides the Corvair and the VW swing axle designs. In some ways I liked 'ol Ralph but sometimes I think he went overboard a bit. 

Even though the design changed to eliminate the problem of the swing axle, it was too late to save the Corvair. It had other problems. I had the automatic with the little lever on the dash for shifting. It had reverse, neutral, drive, and low. There was no park except for the parking brake. I learned early to turn the wheels into the curb if there was one and I was on a hill as the parking brake was also not that effective. I also was driving home and it was a freezing rain. I got of the more main road onto a road that was not so well maintained and started fishtailing. I tried to correct and it kept getting worse. I ended up doing one or two spins (I don't remember how many), and went off the side of the road into a snowbank. It was like I parallel parked half-way between two telephone poles. When I went into the snow, it knocked the throttle connection loose so I could not move. My dad came and figured out the problem so we reconnected the linkage and we got the Corvair moving again.

It was also slow to get moving. The worse was that the engine leaked oil, so I had to keep a window open so I could let the smog out. The defroster did sort of work as did the heat, but the smoke coming in with the heat was not very nice. It could not have been good to breathe either.

I got a brand-new Pinto to replace it. I am just glad I did not get a Vega.

Talk about out of the frying pan into the (literal) fire. 'Ol Ralph went after the Pinto even more than he did the Corvair. I recall trying to fix the brakes on my girl friend's Pinto, ended up replacing everything in the whole system and it still wouldn't stop right. At least it didn't have an aluminum block. Or you could have bought a Gremlin. Very unfortunate name choice....

My first new car purchase was also in that timeframe, a first gen Honda Civic.  Very nice, efficient, and reliable little car. 

I got a brand-new Pinto to replace it. I am just glad I did not get a Vega.

During my college years at Michigan State I was forced to sell my '69 Corvette as I was putting my own way through school w/o any outside help.  I replaced it with a 74 Gremlin, still remember the numbers, sold the Vette with the optional hardtop for $ 4000  and purchased the Gremlin off the lot for $ 2902 total price!  Wish I had the Vette now, worth quite a bit.

College was affordable then, 4 years set me back $ 11,000 - not bad for a 4 year degree.  Nowdays that only pays for 1/2 year.  And yes the Gremlin . . . . even though it wasn't a eye turner like the Vette, it was more dependable as I was always fussin' with the Vette.  Had the Gremlin for 8 years, 125,000 miles and not 1 issue the whole time.  Sold it for $ 800 so the price/year of Gremlin ownership was only $ 262.50.  Even abused the Gremlin as I would get 9 of us in that thing going to church 3 each in the front and back seats and 3 in the rear with legs hanging out with the hatch window up.  Pretty funny sight!  Should have put air shocks on as it would hang pretty low in the back. 

Neither were suitable for towing.