We had an R-Pod 173 and it was easy to pull with a small truck like a Tacoma. The gas mileage was still bad, due to the wind drag effect. The bathroom was very tight and storage was limited. It ended up being too small for my husband and me, but if your parents don't mind tight spaces, they'll be alright. Because my husband and I like to have a couch to sit on, I'd pick the 175 over the 177. Again, it's personal preference. Have you looked at the Funfinder X? It's got more room and a full bath, but it's still light weight. I'd recommend the model in my signature, but it's been discontinued. Tracer has other small trailers you might want to consider. Good luck in your search!
Beth and DH
3 Grown, Non-camping children
Maddie (westie/bichon mix?) and Nash (morkie?)
2012 Aliner Classic
2010 Nissan Pathfinder
My wife and I have had an RPod 175 for about a year. It fits us well and tows fine behind our 4.0 Toyota Tacoma Prerunner (10-13 mpg). The RPod 175 is realistically only good for two. The slide out couch does make down to a bed, but is rather short for any adults. The bed is a short queen size, so most people who approach 6' or more would not be comfortable unless they sleep in the fetal position. The mattress in ours is not great for comfort, but a foam topper took care of this.
The bathroom seems fine to us, but we are very short of stature. It could be tight for larger people. Some don't like that it is a wet bath, but not a problem to us. All three holding tanks are good size for a small trailer- -30 plus gallons each.
We did not buy our trailer with axle risers, but had them installed, as we found the RPods sat too low for even normal towing. I wouldn't want one without the risers. We really like our RPod and have found the build quality much better than most of our previous trailers-Aljo, Chalet, Layton. The quality is not quite up to what our Arctic Fox was, but still very satisfactory to us. The MPG makes a trailer very similar to our 175-a little bit more roomy inside, with some different features, but we didn't feel the build quality was as good.