When we had a tt I put a spare tire cover from a car trunk, tire sized round fiberglass plate, underneath the spare and covered the top with an elasticized tire cover to keep the road dirt off of it. It was mounted on the underside of the A frame. I removed and cleaned it regularily.
With the 5th wheel I lower the spare and cover it from the bottom with an elasticized tire cover and raise it back in to position. On our 2008 5er it never collected dirt or water, I inspected it regularly.
On the 2012 5er I only had it on for about a week on the first trip and experienced lots of rain. When we got stopped and setup the neighbor asked me what that domed shape thing was at the rear underside of the unit. Said I don't know. Got down on all 4 and commenced to laughing. It was a huge beach ball sized tire cover stretched almost to the ground. I pierced a hole in the center and drained it. When we got home 4 weeks later I put a large grommet in a hole where I drained the cover. I check it regularly and inspect the tire to make sure it is staying dry.
Married To The Navigator
2005 Red GMC Duramax 4X4 HD Crew Cab
4" Magnaflow, Banks Air Intake
Firestone Ride Rite Air Springs