Ongoing Electrical Problems

This thread is old so not sure if I should continue it or start a new one.
I resolved (at least I thought) the issue on no house power via generator by bringing RV into the shop. I was advised ATS motherboard had gone bad. It was under warranty so had new ATS installed (same PD5100 Series).
No issues during the year with local trips. However, made same holiday trip from TX to VA (25hrs/one way) and electric problems are back with a few extra twists.
12hrs into the trip we made a stop for gas. It was our 2nd or 3rd stop and no prior issues. Shut down Gen, gassed up, and got back on the road. When we fired Gen up - no house power. We had shore power when we arrived in VA and AC power was restored.
Like the previous year I went thru possible issues: 1) confirmed no blown fuses in converter and reset all AC breakers, 2) switched genset breaker to off and back to on, 3) switched 50amp breaker in house battery compartment off then on. No results.
Rather then wait for return to TX, I disconnected shore power and accessed the ATS then manually engaged the relay with Gen running. AC power to house was restored but according to Progessivedyn troubleshooting guide that meant circuit board in ATS failed and had to be replaced. There was no evidence of any wiring damage in ATS. We stopped and restarted Gen several times and relay continued to engage. Also, with Gen running we connected shore power and Gen relay disengaged as it should.
On journey back to TX was initially good. However, about 1/2 way back we lost 12V power. Gen was still providing power then at about 3/4 way home it also stopped.
Now home and trying to figure it out. Everything works on shore power. First want to address no 12V power. DMM confirmed house batteries fully charged. In review of Ed Felker's User Guide, I've found the Use/Store switch is not making "clicking or clunking sound". He mentions the relay contacts may be frozen in the store position and notes the relay is in the black box in driver's side storage compartment. Is this the fuse box, Thor Part 0300224, in that compartment? Pics attached. Or is it somewhere else?
