Sit back and read this while I tell you about 40yo RVs....
Start by inspecting all the rubber parts. Assume they are all aged out and you will not be disappointed. This is not limited to tires that have an 8 year life (from the DOT date), but also the belts, coolant hoses, brake lines and door seals. So, count on flushing the brake system while replacing the rubber lines.
Expect trouble with the fuel system as it was not made to be compatible with the current crapahol at the pumps. All rubber fuel lines need to be replaced with alcohol compatible materials, but so does the engine fuel pump as its diaphragm will fail. You can find local people that know how to rebuild the carburetor with alcohol resistant parts. Plan on getting that done.
Now, all that said, the work outlined above is not expensive but it is labor intensive and will cost lot if you farm it out.
Buy a good grease gun and two tubes of Valvoline Syn-power. The grease second only to Caterpillar Desert Gold, but it is more available. Try to over grease everything.
Search the web for the manuals for all the installed stuff. Most of it is out there, but it isn't always easy to find.
If you see any evidence of water intrusion, inspect it carefully. My Dodge was a Cayo and they had no structural wood and neither does the current GMC. But both had some water damage. Spend some time on the roof and caulk anything you suspect. If the caulk you see is not solid, carve it out and re-do it.
If you search the web, you may find a replacement window. Or, you might have to make a road trip to the two RV surplus places in White Pigeon. One is Bontrager's and the other Johnson's or some such. There are an number online places too.
Does this guy know about this?
I had a 78 Cayo Motorvator. That was on a Dodge 300 chassis. I barely got to use it.
Now we have a 73 GMC that I have been able to get ahead of and have put 60k miles on it in the last 12 years with only once on a hook. I'll take it.
Get in touch with the owners of that era Coachmen. They were a good build, but any other people's experience will be valuable. Get the book for the Dodge chassis as soon as you can. You will need it.
Lastly, if you PM me with a phone number and good times to call, I can hopefully same you some grief.
Matt