With a multimeter and a jumper wire with spring clips you can easily test this. Check the ground pin in the Ranger front socket against a good ground in the truck. If no resistance is shown, that ground is good. Next do the same in the RV. Check the ground pin in the socket on the RV rear against a good ground on the RV. Again, if no or little resistance is shown, then check the ground pins in the umbilical cord.
Since you say the lights work, just too dim, it's most likely a ground issue. You could also use the multimeter to test and see that you are getting full voltage in the RV socket and at the tail lights on the Ranger.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI