You mentioned that the chassis battery went "flat" on a previous outing. By that I assume you mean dead. If thats the case, start there. I say that because you have traced and checked all the terminal connections from the battery posts, to the main power lead to the starter motor. If you can get it started, drive it to advanced auto, autozone, etc. and have them load test your battery.
But, before you take that drive, do a little battery shopping before you go to see who carries the replacement AGM battery, in case you need it.
I say this because, when you killed the battery, your battery was partly damaged, it's just part of battery 101. And, always look at the simplest thing first. Connections are good, check the battery. In case you go to advanced auto, here is a coupon link. You just need to have someone order online for you, if you find out it STB, while you wait at the store. $50 off Advance Auto Parts Coupon