We had a drippy temperature / pressure relief valve on our '87 (tank and valve are original to the trailer) and I actually got out the old owner's manual and read it
. The fix, for at least our drippy valve, was the following procedure as outlined in the owner's manual...
1 - Let water come up to temperature.
2 - Turn off city water and on board water pump.
3 - Manually open the temperature / pressure valve and let it open until no more water drains from it (be careful here becasue the water is obviously going to be HOT).
3 - Close the temperature / pressure valve.
4 - Turn the city water or pump back on.
I think the manual made mention of an expansion air bubble in the tank and if that air bubble wasn't there the valve may drip.