We recently sold our 1991 Beaver which we had for 19 years. The 1990 through 1993 Beavers have been called the beast coaches that Beaver ever made. A friend bought ours, he's delighted and now full time in it.
At a price of 10K, and if you're prepared to do a little work, I'd go for it. It may be a little slow, but it will get you there. We had a continuous average of about 8.5 mpg and did as good as 10.4 with our CAT 3176 in a 40' Marquis. The mileage you stated sounds to low for that coach.
Our's also had the problem of clear coat pealing all along the curve between the roof and sides. We just lived with it. Just make sure the seams are well caulked and don't use silicon caulk. You can try this treatment on the pealing clearcoat. did the ZEP (Red Max Pro) treatment today on my 98 Discovery
Also see Bodywork shine - Red Max Pro 3 no longer avail - ZEP is same product :-)
First thing is change all the fluids, replace any tires over 7 years old, make sure the transmission air shifter is in good working condition, and enjoy the coach.
I've got quite a collection of Beaver parts sources etc. Send me a PM if you get the coach and I'll be happy to send you copies.