You don't have to take the cracked wheel off the camper to figure out the correct rim. Just look at the spare tire. The same rim size should be stamped on the back of it too.
For an immediate response, I'd check with my local RV dealership, or wherever you purchased the Montana from (if from a dealership). Second, I'd check with Discount Tires. Once you have the numbers off the back of the rim, you can call around different tire shops in your area and see what they say.
Another option, if anything exists in your area is an RV salvage yard. Google is your friend there.