My specific concern is that I'd like to relocate weight aft in my 179 to reduce tongue weight when I'm heading out (with a full fresh water tank which is forward in the 179). But coming back, that fresh water has now gotten dirty and moved rearward, so I don't want to end up causing trailer sway inbound in exchange for heavy tongue weight outbound. 

So, particularly for you folks with trailers with aft fresh water tanks, and who have observed sway, what were the trailer and tongue weights, or % tongue weight (if you know), when that happened?

Thanks in advance!

All that being said, we have a lighter than average trailer and don't bring a lot of heavy stuff along.  Extra propane is kept in the truck bed, along with any other heavy stuff, so i've never really worried much about the the load.  I practice the Zen of trailer loading, trying to keep things in a kinda-sorta balance though reflection, meditation, and insight.  When everything is finally hooked up, I look at the relative position and angles, and it it's aesthetically pleasing and gives me inspiration to start the motor, off I go.  If something doesn't look right or feels odd when I start out driving, I'll go back and take a second look at where I put things.  With the exception of the propane kept at the tailgate, I try to put heavy stuff toward the front of the truck bed.

This is a totally unscientific method of trailer loading.  Some day I'll figure out where a public scale is and when I'm loaded as heavy as we ever get, I'll go there and make the ultimate test of Zen vs. objective measurement.

If your fresh tank in the 172 is just aft of the axle and the gray is just forward, shifting a full 30 gallons from one to the other probably changes the tongue weight by only about maybe 50 lbs. So I'm guessing that  if you're running light and you have a propane tank and a battery there you're still over the 11% on the tongue that FR lists for the empty 172.  Unfortunately for me, in the 179 the fresh tank is farther forward so the shift winds up being more like a 130 lb change on the tongue. Makes for about a 3.5% change in tongue weight.

We have not gone much beyond seat of the pants "CG balance" similar to lostagain and mcarter. Our TV/TT combo gives us a lot of leeway. We have towed with full FW, and with near-zero FW. Likewise, we have had varying amounts of black and gray, depending on where/when we decided to dump. We have had no issues, but our TV really gives a lot of freedom.

I will note that not long ago, we were traveling through Pacheco Pass, and stopped at the rest area/interpretive center above San Luis reservoir. At the center we spotted an F-150 with a RP-172 in tow parked in the lot, and there was a stream of water draining from the back. Upon seeing that and the owner, he commented that he was transporting the pod from storage to another place, and he'd forgotten to drain the FW tank. He said it made the pod act "squirrelly"; especially when passing big rigs.

So the storel to the morey is tha you need to pay attention to the weight and balance, and each pod/TV combination is going to have its quirks and idiosyncrasies.

Pacheco Pass is a notoriously windy place, I can see why someone could have issues there even without the trucks. 

We never had a microwave [which would be pretty much over the axle] and took out the tele and heavy particle board table [aft of the axle].  Food tends to get packed above the galley and fridge, so it is pretty much centered over the axle.  We have a heavy cast iron dutch oven along with the drinking water under the dinette seat.  In the storage area at the far back, we keep probably 40 or 50 lbs of stuff such as tools, cables, charcoal, wood for blocking, and a jack.  

Forward, we tend to have our clothes up on the shelf I made from the old 172 upper bunk and the new shelf I just added.  Below the bed, we have probably about 100 lbs of stuff, such as the solar panel and controller, some folding chairs, sheets and towels, and the infamous r-Pod eyebrow shade.  Our mattress is a 4" dense foam pad that I'd include in the 100 lb. estimate.  If anything, i'm probably a little heavier on the tongue than your suggested 11%, likely more in the range of 12 to 14% based on my "Zen" of trailer loading.  When we travel with the fresh water tank full, I'll put some of the gallon water bottles forward to avoid being too heavy aft.

What ever we're doing seems to work fine with the truck.  

The listed empty weights for the 2o15 178 are 257 tongue and 2535 trailer, so right at 10.1% tongue wt. All your added stuff should be about 390 lbs and it I estimated distances right about 20 additional lbs on the tongue. So, you should have been at about 9.5% on the tongue. 

So now we have a few data points for rPod trailer sway susceptibility:

7%?  (guy on physics forum) catastrophic sway, trailer stops swaying only when it hits back corner of TV

9.5% (mcarter's 178) sway causes anxious moments but not catastrophic.

11%?(Glueguy's Pacheco pass 172 guy) "a little squirrely"

12% (lostagain's 177) no sway

13-15% (offgrid's and Glueguy's 179's), no sway.

So far this is supporting maybe 11% as a min safe tongue wt rather than the "standard" 10% .  

Lets see if anyone else has any info. 

I've added his data to the list:

This is painting an interesting picture I think. It seems like the rPods are subject to sway at a bit higher tongue weight percentages than the generally accepted 10% number. 

Of course, there are plenty of other factors involved that will help to determine if sway will actually start (side wind from semi's, downhill braking, trailer tire pressures, etc, and even more factors as to whether the sway is damped out before it gets significant (weight and wheelbase of tow vehicle, tow vehicle tire pressures, antisway systems, etc. It's a complex phenomenon and from what I've read so far not as well understood from a mechanical engineering perspective as you'd think. 

Still, I think this information at least gives us an idea when you would want to start being concerned about it, probably anything below 11% tongue weight or so. By 10% sway is happening is some cases. 

That tells me not to try to move very much weight aft in my 179, if any. The 179's lose about 3.5% of tongue weight moving 30 gal of fresh water to 30 gal of gray water, and I'm currently right at 15% with the fresh tank loaded.  that would put me at about 11.5% if the gray tank was filled instead, about as low as I feel comfortable going. 

Thanks everyone.