Gas cap for that vintage of Van should work , provided the RV manufacturer used the filler neck that GM provided with the cut away.
Most auto parts stores will stock what you need, but keep the box and receipt in case it's not right. If you return the same day they won't have a problem.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.