Before messing with the PEX clamps/fittings check the tank nipples.
Your type of valves rarely fail stopping flow when opening.
If they fail it's usually that they don't fully close and leak thru.
When you open hot out valve and get flow then it stops sounds like the symptoms of a check valve problem.
Some rv builders installed check valves even with the 3 valve systems.
Even put them in the cold inlet nipple vs hot outlet nipple.
Next time out to trailer........open WH tank PRV via lever then turn on pump.
If you get steady flow out PRV then cold inlet isn't problem. One less thing to check/worry about. Then you problem is on hot outlet side