The most common key for basements is 751. We were a Fleetwood dealer and they used the door key on a lot of their basement doors.
not the deadbolt key but the regular door key. Make sure you are putting the key in correctly. The uncut side goes to the center of the lock and the cut side goes to the outer edge. Some of the locks had a tiny number stamped on them. Look closely for a tm314 or similar number on the lock. It will be on the shiny center of the lock where the key goes in. You can also look in the nice folder that came in the motorhome. There is usually a page that gives all the key numbers and which lock they fit.
1998 Pace Arrow 35 ft. F53 Ford V10 2012 Honda Civic toad
15 year motorhome service manager. 3 popups....2 travel trailers....5 motorhomes....loved them all.