Hot water tank - 12 volt issue

[CampMacKay]

My first new post in 5 years of ownership, this is the first problem I've had that I couldn't figure out and fix.
I have no hot water either on gas or AC, it worked ok on our last trip. The by-pass valves are in the correct position. There is 12 volts to the water heater on the gas and electric remote switch wires (yellow and white). No 12 volts on the "green" wire (shown on the schematic on the water heater). Gas is turned on, fridge and stove are working ok on it.
What I have done so far:
Checked all wiring, as stated above, checked all fuses (ok), re-set water heater breaker, pulled the heater out about 4 inches to try and trace the wires to it. Forgot to turn off the gas switch, tugged gently on the wires from the heater to try and see where they went. All of a sudden we heard the tell tail click click click and the heater started on gas, thought we had it firgured out, not! Tugged on the wire that goes to the error light and it came out from where it was connected. So then we pulled the heater out further and found a bundle of wire nuts wrapped in electrical tape. Unwrapped the bundle and re-connected the error light wire and re-fastened all the other connections securely. While doing this I also checked for 12 volts on the three wires that should have it. Both remote switches were good, but there was no voltage on the 12 volt input wire. The heater at this point would not work. The 12 volt input wire is connected to 3 wires in the wire nut, so I thought there must be something else not working in the trailer, but everything checked out ok. I have a fuse that is labeled "trunk" and one labeled "appl", if I pull the appl fuse the fridge shuts off.

Does anyone know which circuit the water heater might be on?

I pulled the wire from the trunk terminal and attached it to the bedroom circuit, made no difference to the water heater. I gave up at this point, until I can get my own tools and supplies.

My next step is to pull the heater out again and run, temporarily, a dedicated 12 volt wire from either the battery or converter to the 12 volt input wire and see if it works. If it does I will have to try and trace the original wires for a break, or wire in a dedicated wire from the converter or a light or some other 12 volt source. From what I see the wiring goes up across the ceiling from the control centre, I don't know where the 12 volt wire from the converter runs, I'm hoping it goes along the wall or floor. Did I mention the water heater is surounded by cabinets, and I can only access the back through 3 small drawer openings, very difficult to even see, or get your hands near it.

If anyone has any ideas where to look for the wiring, knows which circuit the heater is on, or can think of anything I didn't check, please help. A cold showered wife is NOT a happy wife!

[hondavalk]

In outside compartment is a thermal fuse (comes from circuit board & goes to t-stat----has plastic tube on it)
When it trips (one shot device---has to be replaced) it shuts-down ALL dc power so neither heat source will function.
It trips at 190*F caused by flame blowing back out of combustion chamber (spider webs/mud dabber nests etc.) Mine quite working because of corrosion. After cleaning the blades on the wire connectors it worked. Thats at least how it is on my Attwood WH

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You can remove it and plug brown wire from circuit board onto terminal of t-stat...then turn ON/OFF switch OFF to reset fault then turn back ON to test fire. (Use propane so that you can hear/see main flame light off.

[CampMacKay]

I gave the fuse a look over, and saw nothing abnormal.
How do you tell if the thermal fuse is bad, check with a multi-meter?
I couldn't get a voltage reading from the trailer wires that feed the heater, maybe I need to ground it to the frame, not the heater, when checking though.
So if I just by-pass the fuse this will tell me that it has blown? The gas burner did fire up while I was pulling on the input wires. Where I pulled, they are nowhere near the fuse, so I didn't give it much thought.
I will definitely check the fuse, more closely, next time I can get at the trailer, thanks.

[CampMacKay]

Went yesterday to do some more investigating. Found that the 12 volt Shown on the schematic actually had a - in front of it, therefore it is a ground. Never seen a ground labeled like this before. Checked the thermal fuse, it is ok.
Determined that there was no voltage on either terminal for the fuse on the igniter board, luckily there is an RV dealer 2 minutes from where I dropped the trailer for a few days, so I take the board there and have it tested. They fired up a heater in a unit on the lot, then plugged in my board, nothing happened. So the board is bad. Ordered a new board, which should arrive today. Heading out Saturday morning for a week in the woods, so hopefully the board will be there and it fixes the problem.
Happy camping...😎