I am always amazed about perceived repairs to a simple problem.
1. I have NEVER in 41 years as an RV Tech had a specific complaint that the OP posted
2. WITHOUT the air gap in the Water heater, the pop off valve will leak once the HOT water in the tank reaches 150 psi. Without a INLET cold water Check Valve that pressure will extend to the cold water piping.
3. An accumulator tank is not needed for this specific problem. It may diminish the problem but you need to FIX the cause of the problem
4. Establish the Air Gap in the Water Heater Tank
5. Install a check valve on the Water Heater Cold Inlet. Doug
To restore the Air Gap
Turn off water heater.
Turn off incoming water supply.
Open the closest hot water faucet in the coach.
Pull handle of pressure-temperature relief valve straight out and allow water to flow until it stops.
Allow pressure-temperature relief valve to snap shut, close the faucet then turn on the water supply.