I have a suncamp TT and just tested the lights before next weekends camping trip and all seems to be good on the lights apart from the right hand side side light.
This is the side and brake light. Brake light is operating correctly but no side.
Aren't the side and brake light filaments in the same bulb? If so it may be either that the bulb has blown or just surface corrosion which the bulb and terminals just need a rub with wire wool and some Brasso. When you replace the clean bulb apply a smear of WD40 which will also clean and protect the contacts against damp.
I had a similar problem last year before setting off. None of the lights seemed to be working, and the reason was because on 7pin plug some of the pins had been push into the plug, thus not making contact!
You might want to check that all your pins inside the plug are where they should be.
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2015
Somewhere near Salcombe - May
Camping Bel, Vendee - jul/aug
Had this kind of problem with ours.
Took the whole thing to bits to find out it was just the bendy metal connector that the bottom of the bulb should be making contact with needed pushing up a touch.
Good luck.