Solved
I did it like you said, Wiz. And I was able to get it to work intermittently.
But that obviously wasn't good enough, so I cracked 'er open. Not easy, but I was able to get in there and check out the relays. First thing I noticed is how clean the points were. No signs of arcing at all. Makes me wonder if my grandfather-in-law either unplugged from shore first before ever starting the generator... or at least was good enough not to have any major loads running before starting. He was pretty sharp, so both of the above could be possible.
But what I DID find was something funky with the cantilever action of the generator input relay. The house relay worked as one would expect, but the other one acted like something was broken at the fulcrum or something. But it was very difficult to see what was going on in there.
I messed around with it a bit and it suddenly started rocking properly. I messed with it some more and got it to foul up again.
After maybe a half dozen of these back and forths, I was no longer able to get it to foul up.
I sure wish I could have seen better and figured out exactly what was wrong, but... it's working as it should now and... I'll just have to keep my fingers crossed.
But at least I know where the problem lies. If I run into trouble while out on the open road again, I'll break out the tools and crack 'er open again.
But I'm so happy I was able to get this far. Replacement... or even landfilling it and wiring up an old-school, manual outlet... were much more labor intensive options. I still work 60 hours a week and the stealership would probably want an arm and two legs for any of those options.
Thanks everyone!
EDIT: Additionally...
Parallax got back to me on the question of whether or not the ATS501 is a direct replacement for the 5070 vs the ATS503.
"Hello Kendall,
The easiest swap by far would be the ATS501.
The ATS503 needs wire slack to be able to be connected and the chassis on the ATS503 is larger.
You will need to go with the ATS501.
Have a wonderful day Kendall,
Bruce Jones
Warranty Dept
Parallax Power Supply"
