We have a 2007 Jayco 20BH. At 20' long and 6000lbs GVW, its probably about at the top end size wise of what you are looking for. For us its just right - big enough that our family of 3 plus the occasional MIL fit comfortably, but small enough we don't have to worry too much about it fitting. We primarily boondock for 2 night trips and have a great time.
Only catches:
Mod the water tank overflow by adding valves. On some inclines, water will be forced out and you can loose up to 1/3 of your water. Cheap and easy mod.
You may want to flip the axles for better clearance.
