Call the tire shops and ask if they can handle your RV. When you ask, find out if they can handle more than just replacing tires. I had used a shop when I got the clipper to put new tires on it, but when I needed some other work done with the bearings or some such, they couldn't handle that work with the RV. I ended up finding another shop that can handle all my wheel related work.
Also, many local shops will match online tire prices, so once you identify the tires you want, check online for the best price and then ask your local shop if they'll match it.
You should also check the age on all your other tires - it's possible they're all ready for replacement...