You need to stop thinking 'low maintenance'. Think 'yearly maintenance'.
You need to do a yearly roof inspection and put that flowing caulk on Anything that looks minimally questionable + inspect anywhere else water can penetrate the exterior shell (regardless of what its made of).
It really isn't all that much work, tho some folks physically cant get on the roof. In that case, get a trusted friend or even pay the RV dealer to do the annual.
If you do that, the chances of water problems are minimal at least.
We started with a 18.9 TT (just the 2 of us + large Akita) and it was plenty. For longer trips, we decided that we needed a separate bedroom so if one of us needed a snooze and the other wanted to do something else on a rainy day, we would have the comfort of an 'isolation' room. So we went larger. Lots of good advice above. Main thing to rem is Floorplan is Everything and everyone's needs in a floorplan is different. Get the one that works best for You.