If the seller will not let you test any of the appliances, plumbing, etc., walk away. No matter what reason they give you: hoses put away for winter, propane tank empty, etc. Tell them it's no deal unless you can test everything. Doesn't matter if private seller or dealership.
Turn on lights and water pump with just battery power. Have them plug it into shore power and check the outlets, etc.
Run water through the faucets.
Light stove/oven and water heater.
Ask the seller to cool down the fridge before you come to see it. While you will not be able to necessarily confirm that it all works, at least you'll know it does get cold. Some take a while to cool down.
1975 American Clipper RV with Dodge 360 (photo in profile)
1998 American Clipper Fold n Roll Folding Trailer
Both born in Morgan Hill, CA to Irv Perch (Daddy of the Aristocrat trailers)