"average condition" is a very relative term. It usually represents much better condition in the mind of a buyer than of a seller.
One thing to be aware of on a '73 is that unless someone has added it, you'll get no gray tank. That means schlepping around a blueboy unless you are in a full-hookup site, or adding a gray tank.
You can assume that the original axles are ready to be replaced. That's not a terrible thing since you can easily get the new ones with all new bearings and brakes already installed, but it ain't free.
If there's nothing wrong with the "original interior" or the plumbing, and the refrigerator works, and the body is straight, and the tires are of decent quality (you already specified that they're new) and all the glass is original and in good condition but it's cosmetically just average, total off-the-cuff number would be around $4000 (maybe $4500 with a working AC unit...you're in WI, it may not have one at all.) This assumes no unusual problems with the floor... it's likely to have a weak spot or two but at that price I'd expect it to be able to travel for a few years before needing any subfloor work.
There's a whole list of things that could drive the price up or down from there. I see all sorts of gutted shells on the market for $4k, so sellers have different ideas than buyers.
- New battery
- modern converter/charger
- PEX plumbing (properly done)
- new fixtures
- window covers for the unobtainium pano windows
- evidence of regular use and maintenance
- very soft floors, visible rot at perimeter of subfloor
- original plumbing
- signs of hard use/damage to plastic bathroom fittings
- significant shell damage
- signs of hitches, baskets, racks etc. attached at back bumper
- poor/no safety chains, breakaway switch, etc.
- poor condition of the plastic taillight housing (big chunks missing, etc. that would make it hard to repair.)