hot water issue

[JSL3]

On my 1986 Pearson 28-2, I’m having hot water issues. The hot water gets scalding hot after running the engine for awhile. The hot water is bleeding back into the cold side of the pressurized water system. So after the engine runs, all I have is hot water from either faucet (head and galley sink) both hot and cold sides. I have the 3/8” grey Quest (Qest) piping. This issue is relatively new meaning it didn’t used to be this way a couple of seasons ago. It has slowly been getting worse over the past 2 years.
The Raritan H20 heater installation instructions show a check (backflow preventer) valve on the cold side inlet to the H20 heater (which I don’t think I have currently in my setup). I am planning on ordering a new Quest check valve like this from a RV parts store.
So 2 questions.
1. Does it sound like I’m headed in the right direction to solve this HOT water issue?
2. Does anyone know what kind of pipe dope to use on quest fittings? Is it just normal everyday plumbing pipe dope or perhaps Teflon tape? You can see some white stuff on some of the fitting threads in the picture.
Thanks in advance.
Here is a picture of the current setup.

[treilley]

Yes. Backflow preventer between tank and Tee on the lower hose. Also good to add a mixing valve to temper your hot water. engine likely running at 160° which is dangerous hot for domestic supply.

I use Teflon tape to lubricate the threads and do not over-tighten. The thread does not seal the connection, the seat does.

I get my parts from these folks. Very friendly and helpful.

http://www.mobilehomedepotmi.com/par...tings_qest.htm

[JSL3]

Thanks for the response, Tim.
I like the idea of the thermostatic mixing valve, I'll look into that.
Teflon tape works, I have that in the basement.
And thanks for the link, I'll order from them.