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Backed up pressure under the sink is NOT the check valve. Your not very clear but it sounds as if you may have lost your air bubble in the hot water tank.
To restore your air bubble remove the hose from the park fitting and the trailer and drain all the water out of it. Reconnect it tightly. Turn the park water back on. Open the hot tap and let it run. If a bit of air spits out you're OK. If not repeat the proceedure untill you get a few spits of air in your hot water.
The air bubble in the tank is very important. It allows the water in the tank to expand when heated. If the air bubble dissolves, the heated water spikes the pressure in your system hopefully just leaking out of a fitting. Worst case it will blow a line, the tank relief or the tank itself...whatever is the weakest link. Adding an air cell to the system prevents this from happening.
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