Assume all fluids and filters for the engine and generator need replacing. Don't forget the transmission and differential. Also chassis lube including bearings. Tires and belts you already mentioned.
On my 40' diesel pusher including tires this would cost under $8000; possibly as low as $5000 (6 tires are a BIG part of that) but probably more like $6k-$7k. However if that is what it costs you to get a motorhome you are doing well!
Having done all that you now have a known baseline for future maintenance.