Good point!  Always read the details.  For example: I have CAA RV+ coverage - the base level doesn't cover the trailer... or the motorcycle.

There's absolutely no funny-business going on.  You can feel comfortable to sign-up and receive a year's worth of roadside assistance with no further payment or commitment.  Just before your year is up, you'll receive a renewal reminder, which you can feel free to decline.  Plan prices and details are available on their website - they hide nothing.

Over the years, I've had CAA (Canadian Automobile Association), Good Sam and Coach Net.  They all offer the same essential service for your trailer, and all respond in a timely manner when you're stranded at the side of the road, but there are differences in the number of claims/amount of claim that you can make, how far and to which repair centres they'll tow your disabled vehicle/trailer to, additional/family vehicles that may be covered under the plan as well and in the extras (discounts, advice phone centres, etc).  Personally, I've stuck with CAA because I'm also covered with my motorcycle, I make better use of CAA/AAA discount travel (e.g. flights, rental cars), attraction tickets and group discounts (including insurance for the trailer).  But, really, if you decide to pay for a roadside service, you should look around first and see what best suits you.

-edit-

I'll add in my favourite roadside assistance story.  Its a simple story.  I didn't have an accident or an emergency.  Instead its a tale of happenstance and (perhaps) a little bit of stupidity on my part:

On a particularly rainy weekend, I was camping at a favourite campground in a site that sloped away from the road.  As I was backing in, the wet grass fairy quickly turned to slick mud and it wasn't long before I discovered that I'd managed to get the trailer stuck.  I rocked the vehicle+trailer back and forth, tried a few different angles and just made things worse.  I got out and took a look around; the mud was only about ankle deep.  I sighed and thought, "alright, I'll cut down some branches and put them under the vehicle's tires for traction."  But I knew it was just going to turn into a muddy mess before it got better.  As I was walking back to the trailer to grab a hatchet, I thought to myself, "wait a minute: I pay for someone else to deal with these problems."  So, I called roadside, they showed up 20 minutes later and they winched my r-pod out to the road.  Quick, easy and painless - and I didn't end up covered in mud.  I got a new site and enjoyed the weekend.

And that's why I like having roadside assistance.  Over the years, I've changed-out flat tires at the side of the road, I've used a coathanger to open the car door when I've locked my keys inside, I've walked 5 km up the road to buy a gas can (and gas) at a station and then walked back to fill my empty tank, I've asked a friendly passerby for a boost for my dead battery, I've pushed cars out of ditches when I've slid off an icy road, I've fixed electrical problems at the roadside, I've jury-rigged motorcycle clutches when the cable broke and banged-out bent sprockets with large rocks so that we could ride back to civilization.  And, if I have to, I'll do it all again.  But, sometimes, its nice just to call someone to come out and do it for you while you sit back and enjoy a cup of coffee.