I'd get whatever decent new tires I could find at a reasonable price that are of the right size and load rating. Others here would of disagree with this and insist on getting one or a few tire brands. I may be a bit naive, but so far it's worked out fine for me--I've never had tire woes due to defective tires, only due to defective roads or driving.
When I got my tires replaced recently, a local tire shop did it for $160 per, mounted and balanced and installed. They're not a well-known brand, but so far seem to do the trick just fine.
Some shops don't like to work on larger vehicles. I found it useful to use the tirerack.com website's "find an installer" option and looking at the information provided by local area shops; most that have restrictions on vehicle types put that information in their comments. (This is incidentally also a good way to get a feel for tire prices, though do bear in mind if actually ordering through them that you'll be paying for shipping the tires, which adds up. It might be a better deal overall to get them through a local shop even if the base price is more.)
For most people, motorhome tires never wear out in the sense of the tread becoming too thin, but rather need replacement due to aging. As such, unless you'll be criss-crossing the country multiple times, it really doesn't matter much what the mileage rating of the tires is.