We owned a 2009 Jayco Greyhawk 31FS for 8 years. The last few years we camped out of it seasonally (full time during the season). We had a handful of issues handled under warranty. The only real problem we had that was re-occuring was the tri-door shower doors kept failing. We had it addressed under warranty. That fix lasted a few years, but then they failed again. We ended up removing the doors and just using a shower curtain.
We had to replace the IOTA Converter/Charger at 5 years, not really Jayco's fault. The fuel pump for the generator was replaced under warranty, not really Jayco's fault either. There was a handful of other little things that we handled, but overall, we were very happy with our Greyhawk.
If you do go with Jayco, one thing I would suggest before using the motorhome is put teflon tape on every water fitting. The folks at Jayco apparently never heard of it. We had several fittings leak in our Greyhawk and in our current Jayco Bungalow TT. Adding teflon tape to the threads cleared that up. A bouncing motorhome causes all sorts of things to work loose. If someone levels the water pump on while traveling and a fitting works loose, you'll have water somewhere.
Edited to add: Our Greyhawk had 2 slides and was heavy. We got anywhere from 6.5 mpg (in the mountains of GA and SC) to 9 mpg (driving 66 mph on flat interstate highways). Average was probably 7.5 mpg - 8 mpg at the speed limit, which in our neck of the woods is 70 mph on the interstates.
-Michael