Yup, these things happen.
A few thoughts based on the pics and comments.
When looking for chassis parts, remember that is a a 1981 P30 first, with a 7.4 Big Block, and then a Motorhome chassis. You will find that will get you on the right parts page and allow the parts guy to locate what you need.
The transmission is normally a TH475, which is a slightly HD version of the TH400, but with straight cut sun gear and a parking drum brake on the back.
Brakes should be "JF9", which is to say Hydroboost operated, discs on all wheels. The rotors on the front are slightly larger than the rear, but they use the same caliper and same pads... and none of that is expensive to replace.
RockAuto is a great source of parts at good prices, but sometimes you local autoparts place can come close, and its either right there on the shelf, or 24hrs away.
If I were you, here would be my priority list when it arrives "Home".
1/ Get her watertight if she will be living outside. That means sealing over the broken and missing glass obviously for starters. Then I would pull up/inspect all the floors looking for staining or wet ply/OSB. This will tell you what the story is or was. Here are the favorites leaks places. A/ windshield/cockpit side windows. B/ Joints on front and rear end caps. C/ Roof piercings, like AC Units, Fan units, and my fave, the seals on the grey and black tank roof vents.
2/ Find used glass for the cockpit and side windows that are missing. Windshield glasses and seals, will be new I assume.
3/ Go thru the Motor/Trans/Cooling/Fuel/Brakes/axle replacing all fluids/filters/rubber hoses, unless you have evidence of more recent work. As you go, look for leaks and other clues from the past. If I was dropping the trans pan, I would never replace the stock tin pan and would go with a deep aluminum or Derale steel pan($120) with a drain plug and magnets... altho this will probably involve a little trans Xmember clearance...
4/ New tires... manly people have swapped from the original 8R 19.5 to 225/70R 19.5 without issues.
5/ Replace the broken Headlamp units with HID 4x6 conversion kits($85), that will be totally brighter AND remove the electrical load from the notoriously weak GM headlamp switch/wiring.
That should keep you busy for a couple of weeks....