Replacement for broken cover

If it's not a toe stubber or trip hazard, I'd remove it.

If you know someone with a 3D printer (and a bit of design skill), they could make something functional that at least looks more or less similar pretty easily. For that matter, it looks pretty straightforward to fabricate using fiberglass over a plug or in a mold, though that can be a bit messy and irksome work.

Is there another one to compare it with? Mc Master Carr may have something. RV parts are whatever the manufacturer could source at that time to fill the need.
Doubtful that cover is even made any longer, who knows.