We've carried a pair of recumbent bikes on the back of our rig. We bought a couple of small car covers and my wife used the material to make covers for the bikes. Covers do a good job. However, I haven't been happy with the amount of bounce that goes on back there and haven't carried the bikes that way for awhile. Even on an MH, you need to get a very substantial rack to stand the stresses of travel. There was another thread
recently where someone wrote that the rack broke off at the receiver and their bikes were lost. Though the original question was about ladder racks, the reading is interesting.