For several years I've been using a large heavy duty BBQ cover to cover 2 bikes. I got it from Home Depot for under $50.00.
I use some tarp clamps to tie it up tight.
Just measure the length of the bikes on the rack and find a cover that will fit.
The less you plan the more options you have.
Currently with a 26' Forest River Class "C"
Ex Navy Diver
Ex Archaeologist
Ex Museologist
Actively retired