To my knowledge, this was a custom molding for FMC and since it was made from rubber, most of them have deteriorated over time.  I tried to purchase a new one years ago from RVS and was told that none existed anymore.  Hope you can find a salvage one in from someone in decent shape.  In my case, I decided that to remove mine and leave the area open looked better than the ragged one that was there.  Most of the time it is a nonissue as the fuel fill door is closed and covers the area up.

Leaving it off has one advantage: any fuel spills go down the front side of the tank and down to the ground without running down the finish/paint on the outside of the coach.  Personal experience!

If there was enough demand, it may be possible to have a batch made.  The problem is the cost of the mold as that is a fixed cost and has to be spread over a number of units to make it feasible to do at all.  There is an outside possibility of a generic unit made by someone that would work.  To find it would take a lot of internet sluething.