Trouble with LP water heater

I have an older Attwood LP water heater from the mid 90's and recently I've been having trouble getting the pilot lit. The only way I am able to get the heater going is by holding the lighter underneath the thermocouple. After doing that for approx a minute I am able to turn the switch to the on position and the heater fires and runs fine and the pilot light now has a flame on it and stays lit for the rest of the weekend until I shut it off to go home.
This happens every weekend when I initially try to light the pilot. I've tried turning the gas on when I first arrive and let it run for a few minutes while I open up thinking it might take time for the gas to run from the front of the trailer to the back but this doesn't seem to do anything.
I'm not sure what would cause this and am hoping someone here with knowledge of these things will know the answer and how to fix

Thanks in advance