Questions are good thing!  The answers get tricky!

Mileage: Many get in the 6-8 mpg range.

Insurance: I use Foremost Insurance through my Farmer's Insurance agent.

Problems to watch for: Yes, watch everything!  The coach is three and half decades old so everything is suspect.  You just have to keep an eye on things and start fixing and updating things as needed or you desire to fix/update.

Tire pressure:  I run 85 lbs.  many others run less.  See the inflation tables for your tires and coach weight as published by your tire manufacturer.

The waste system still works on some coaches:  many are disconnected as exhaust system changes were made or some other failure happened.  They also had a tendency to have the inlet tubes plug.  You will have to investigate the system on your coach to get a definative answer that applies to your coach.  While doing other work, I recently discovered my Thermosan pump and removed it.  The pump still works and my grandson thinks it is a great toy!