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I just did a "Failed Circuit board" repair. It was the thermal fuse. The first thing is to confirm the (replaceable) fuses are good and then check the thermal fuse. do this by putting a temporary jumper from one side of the thermal fuse to the other. an Alligator clip jumper works great just so you can allow the 12v to get from one side of the fuse to the other. If you install the jumper and the unit fires up, you found your culprit. As this fuse is a safety device, remember jumpering is only a troubleshooting step, not a permanent solution. (in the below photo, The jumper is the white wire loop with the 2 light blue mark-up lines coming off it) To further my repair, I actually mounted Circuit board mount style screw terminals in the place where the old fuse was, and connected the thermal fuse to the screw terminals. I did this for 2 reasons. Soldering a thermal fuse is hard….the heat from the soldering iron can ruin the fuse, and like some had said, if I'm camping, not likely to have a soldering iron in my tool kit (I do carry the alligator jumper though) in the future, as others have stated, make sure the ALDE is not getting power hits as the camper is plugged in to the Tow Vehicle, Shore Power etc. The way I look at it is every time the 12v converter comes on line, it has the potential to send a surge to downstream components. I took the 12v line going to the ALDE and installed a 2 pole switch in the cabin. The ALDE is switched off until power is stable and I am ready to energize it.

Hey @Oinfront thanks for the words of encouragement. Since my last post we have succeeded in repairing the bathroom sink drain into the grey tank! We thought we were just going to attempt to cut the drain, cap the tank and never use the sink, but once we got under there we actually managed to make the repair so the sink is usable. The trick is to come at from multiple directions: from inside the cabin under the bathroom floor (ala shower drain repair) AND thru the outdoor shower. You can access the sink drain [barely] thru the outdoor shower cavity if you remove the shower assembly - and if your hand and arm is small enough. We also had to go into the adjacent compartment under the refrigerator in order to temporarily pull back the A/C duct for more access thru the outdoor shower. I realize that doesn't make much sense without seeing it. Sadly we didn't take pics, but anyone finding themselves with a similar challenge can feel free to DM me and I'll go into greater detail. The repair process was the same as the other drains, just getting to it is the trick, but we proved it's possible without dropping the entire tank off the frame!

@Bayliss and @MuttonChops are onto something. The LK550 and the newer Alde Speedfit valve differ in where the hot and cold lines connect. See here straight from Alde https://en.alde.se/service-support/tips-and-advice/alde-thermostatic-mixing-valve-tmv-how-to-fit/

and compare the picture in the post above.

Edit: Added credit where credit is due :)

@MuttonChops, [UPDATED (if you have already read my previous response)] - - initially, I though it appeared correct to me, but I based that on how my LK550 mixing valve is configured. However, after looking closely at the markings on the body of the TMV, I believe you are correct!! The TMV has the labeling for "C" (cold water "IN") and "H" (hot water "IN") from the horizontal sides, with the mixture flowing out through the bottom, as depicted by the downward arrow. NOTE: the photo below, which I found online, depicts the opposite side of the TMV, so the "C" and "H" are displayed as oriented for the opposite side, not as the photo depicted by @Voyager.

So, "YES", @MuttonChops, I do believe you have solved the mystery. GREAT DISCOVERY!!!! I believe that is what threw us both off, because we (or at least "I"), was basing my comments on how the LK550 is configured. Apparently, whoever installed the THV on @Voyager's trailer plumbed it like the LK550 (but, just a guess on my part).

The photos below refer to the LK550 Mixing Valve on my trailer.

Unlike the TMV, on the LK550 the fresh (cold) water enters from the bottom of the TMV to mix with the hot water flowing from the Alde tank through the left side of the TMV ("HOT IN" label in photo below), and then the temperature adjusted mixture of the two exits via the right side of the TMV ("HOT OUT" label in the photo below) and continues on from there to the sink and shower valves.

I previously labeled the flow (of all the water lines) inside my Alde compartment to remind me of the direction of flow. However, I now see that instead of labeling the right side "HOT OUT", I should have labeled it "MIXTURE OUT", although it is "hot/warm" water.

I did some quick research regarding the labeling on the LK550 body and located the following info:

Arrows on the valve body indicate the direction of the flow.
KV = incoming cold water
VV = incoming hot water
BV = outgoing warm water

Additionally, the right side of the valve body is situated slightly higher than the left, possibly to help direct the cold incoming water flow upward and to the right to mix with the Alde hot water as the two exit through the right side of the valve.

I included the third photo simply to depict the blue PEX (fresh/cold "IN") water line, which leads to the lower entry point of the mixing valve.