Great post, and great replies!
Welcome to the sickness that has NOTHING to do with AZ dust! LOL!
As one who did something similar, I agree with all the advice given.
Tires are the easy part, many have swappped from 8R to 225/70 without issue.
I would think about your choice of tire for the driven axle... A "traction" tire might makes sense for some people, depending on your home weather and travel plans. Nothing worse than getting stuck on a bit of snow or wet grass with straight grooved tires! Been there done that on a BMW wagon towing a trailer!
The 5 year slumber it has been in is a concern, but add to that the dry heat and dust, and you might best plan to replace the master cylinder, calipers and flexible hoses, and fluid flush. Check the hoses to and from the Hydraboost unit. Mine was/is leaking PS fluid from the feed hose between the reservoir and pump.
Drain all the present Fuel, via suction, THEN replace all filters and rubber pipes. FYI, there should be 3... a large one in the feed between tank and rear electric pump(make sure its preset and working), smaller one on the chassis rail forward of the entrance door, and also mine had one on top of the intake manifold, but if yours still has the quadrajet carb, it maybe inside the carb intake. Buy some spares for the trip, as the junk in the tank will get stirred up. Some fuel treatment like seafoam to help clean the pipes and carb as you drive would not hurt.
Oh, if the genset works, a filter set for that might be worth having to hand.
Fresh engine oil and filter, new airfilter, and a set of plugs and wires might be smart.
Coolant flush, and check all of the hoses, as well as the 70' of heater hose going back and forth to the aux heater and water heater if fitted!
I would also check the condition of the trans fluid and replace that and filter too.
Unless, of course, the current owner has receipts for any of the above work recently.
That way you know where you are.
Then its rear airbags, although I drove mine 1600 miles with my front bags empty because i didn't know it had them!
Window seals are for later, but tape them if you think they are too loose.
Make sure you spare tire cover is secure, as it is virtually irreplaceable. Personally, I would add a safety cord to it... as mine showed evidence of having been dragged by the brake lamp cable...
I flew in for my drive home, so could not bring much beyond a sleeping bag, but I did stop and buy a Sears tool kit, spare oil and fluids etc.
Above all, get yourself a breakdown service...
Having the "Been there done that" hat AND T shirt, we wish you luck..
And LOTS of pictures!