Likely you have had the problem of "dim" taillights or maybe they work off and on?  After checking the bulbs, ground and anything else, you come to the conclusion it is just the light itself.  This may be true as the little piece inside the light that has the contacts that the bulb presses down against may be the culprit.  The little tips that stick up may not be making good contact with the bulb and this piece also has a spring to keep pressure on the bulb.  Maybe you are thinking you want to keep the 'original lights' on your vintage camper, BUT think you may have to change them out for new ones.  Here is one way that you may be able to repair your taillights before scrapping them,  wrtyoung wrote:

"If you have a scotty that has the socket that acts up here is a quick fix. My scotty had taillights that didn't work good. The P.O. had put some extra ones on in the back. They were the cheap generic round types. I knew the problem was the wires and the little board that touches the bulb. On a hunch I pulled the wires out of the generic types and put them in the scotty taillights. The wires had the little round board with the tab to fit in correctly. It also had a spring that forced the board into the bulb. The two wires were in good shape. I put them in and they fit perfectly except I had to ream a little in the wire exit of the original plated piece. I put them it and they work perfect. So if the metal taillight is in good shape but the wires/board is bad go to a auto parts store and look around. You can tell if it will fit by the tab direction. You can see the original taillights and the after market taillights on mine before I rebuilt the outside at PICTURES  PICTURES _2 ."

You can try this and if it doesn't work for you, then maybe you can find a parts-camper to get the taillights from?  At last resort you may have to buy replacement ones.  There are some available that are very similar to originals.  This is one place to look at, Vintage Trailer Supply and there are more.  So, good luck.