Go buy a couple 1/2" PVC valves...1/4 turn, put some pipe tape on the threads where the caps came off, screw the valves on and be done. That was the first thing I did to our 2018, 10 minutes after I parked it in the driveway.
you may be able to replace the seal piece at Lowe's, ... but if not, these are what I replaced that cap with, ... comes in both red and blue(both links below), ... no more unscrewing that cap and then losing it, ...