Haven't done it, but it would be fairly easy. They make a threaded on one side pvc elbow, jut screw that on where the cap is now, glue on a piece of 1/2" PVC pipe the length you want and glue a pvc ball valve on that. The hardest part would probably be finding a way to strap the pipe the the RV frame, Dutchman uses a lot of self drilling screws on the frame to attach things.
2014 Aerolite 213 RBSL
2016 Chevrolet Colorado