Pretty much like buying tires for your car.... Verify correct size.... what brand do you like... ask 10 people you'll get 20 answers as to which is best...... Goodyear/Michelin or no name......
One big difference though is look at the date code on the tires; make sure it hasn't spent the first few years of it's life sitting on a shelf.
Google.com is your friend.... shop around for sales, good/bad service.... make sure the shop knows it's for an RV, some places only do cars, pickups, etc.
But why rims?
'87 Fleetwood Tioga Arrow
If you're in a hurry, you shouldn't be RV'ing