DReiser, On my 2004 Journey 36G, this is from memory, and reading the Body 12v wiring diagrams, I believe power is fed to the tank monitoring circuit board via the 12 volt circuit panel located at the base of our refrigerator. I can not recall how the circuit breakers are labeled but hopefully it will be clear.
I have also read on this forum where some of the tank monitoring circuit boards have failed. You may want to search for this to assist with troubleshooting.
Taking the one panel loose was relatively simple. I found it helpful to first plan a way to hang it once it is loose to allow easier trouble shooting.
I have installed a Sea Level II tank monitoring system and have found it accurate and helpful when dry camping. There are a number of us who have installed this system and provided a how we did it description and photos and each has taken a different approach. You can find my installation
Here
Be sure to look up the other installations as there are some nice installations.
SteveG