I usually only park up for a day or two before moving on and my heat exchanger keeps my hot water plenty warm/extremely hot.
I never bothered switching on the power to the Atwood marine heater, except this last week I was at a campground with hookups (first time ever hooked up) so I plugged in. After a couple of days, the hot water from running the engine was used up/cold so I switched on the power to the unit. After about 3 hours, I turned on the hot water tap and the water hadn't warmed up at all so it looks like the heater isn't working.

Is there a reset button on this unit or some switch other than the main switch near the breaker box?

TIA
Larry

If I understand correctly, you're considering a thermostat for the engine
water heater lines feeding the house water heater. There was never one
from GMC. Presumably the plan was for the house water to reach the
engine's water temperature, or close to it, while there was no other heat
source (or overwhelming and effectively disabling propane if left on).
That's worked well for the past 45+ years except for the need for caution
with engine-heated water.

Unless you're concerned about the high temperature of engine-heated water,
don't worry about it. If that is your concern, look for a "mixing valve".

Ken H.

You don't need to reset the breaker
on the heater every time you start the engine. Only if after running the engine you want/need hot
water heated by electricity.

Larry, I only recently started following this thread but have a comment that may or may not help. I have similar unit and when I bought my coach this year the PO advised me that the hot water heater did not work on 110V. I pulled the panel off and found the reset button needing reset. Did so. Then each time I started the engine it and the coolant got up to temp, it popped (and a bear to get at if you have put things back together). Not knowing much yet about the problem, I ordered a new high low switch and installed it. It did the same thing... blew each time I ran the engine up to temp. Someone, I remember not who, suggested I go back and recheck ALL MY ASSOCIATED wiring was wired correctly to the hot water heater and make sure it is all tight. I did so and did not note any particular errors but it now appears it is no longer popping the reset button. I cannot figure out why nor do I know if it will last.... I do know, though, that the new thermostatic switches are different from the older version.
Rod

Tom,

I did a Google search and found this:

https://www.kuumaproducts.com/products/6-gal-water-heater-120v-front-heat-exch-fr-back-mount

This heater looks just like the Atwood EH-6 I removed from Double Trouble and replaced with a Seaward unit.

<snip>

The high temperature regulator on my Atwood has a little red button to reset it. I drilled a hole through the cover that goes over the electrics so I could reset it without having to take the cover off. See the picture below: (PICTURE LINK DOES NOT WORK)

http://www.gmcmhphotos.com/photos/xantrex-inverter-install/p31262-xantrex-sw-2000-instal.html

If you look in the upper left hand corner of this picture you'll see the wiring schematic for the unit.

Regards,
Rob M.