I'm fairly new to this forum also, you'll find a lot of great knowledge here. I'm responding because I know a couple of folks that have newer Minnie 21 footers. They bought 21's for the size and towing capacity (they tow horse trailers) and are very pleased with them.
There are many sites online with pre purchase checklists. My priorities would be to weigh the rig if I could to confirm the weight ratios. Then I would pay particular attention to the roof and seams,exterior doors and windows, looking for leaks or soft spots. Then on to the coach systems. Fresh, gray, and Black water tanks, pumps, drains and vents, check for leaks. Electrical systems, Refrigerator, generator and AC. Propane systems next, heater, stove and refrigerator. Check for leaks. Then I would crawl under the chassis looking for damage or critters. Then I would finish with the engine, tranny, driveline and rear end. Become as familiar with this rig as you can with the current owner.
Like I said, there are a ton of checklists available and diligence now will save you headaches later. Once again, welcome!