We have the exact same camper bought new.
Here is an old post showing some of the things I've done. http://www.crossroadsowners.com/foru...chen-1231.html
I installed a 1 X 6 cross piece (on edge) under the bed to stop some sagging of the plywood in the center. Also installed a couple more screws into the wall at the hinge area. What seems to destroy the board to wall joint is raising the bed to access storage without sliding the mattress away from the wall. If you don't slide it away it acts as a lever to pull the screws out of the wall.
I also installed an outlet (110v) on the side of the kitchen cabinet near the propane detector and powered it from the spare breaker. That way I can run 2 electric heaters without tripping breakers as they are separate.
We haven't used our furnace much as we are warm weather campers (hate the cold) so can't help on the furnace issues.
We also bought a new mattress from Denver Mattress to replace the original one. I can't believe the difference in sleep.
The only other issue that we have had is the converter quit last summer.
Good luck and keep on camping.