That seems kinda odd… but this is the stuff to use, it’s the only thing I have found that stands up to gas. A gob of this, then screw on your fitting and it should seal up and stay put. You’re on your own for disassembly though…
Lots of times taller drivers bump them with their knees and over time they get loose or sloppy or bent over. Cleaning them well, then wrapping both fittings with electrical tape so they are stuck together, and then a small piece of foam attached to the outside will keep your knee from really damaging them.
Are the caps unscrewing from the tank or are the fuel lines coming off the nipple ends?
If its the caps unscrewing, you could try a tape that can conform to the threads as suggested above. Something thin would be ideal and you make up for the lack of thickness with several wraps around the threads.
If its the lines are coming off, you could try a zip-tie or safety wire to squeeze the barbs on the nipple.