There’s a separate adapter in the trunk (god knows why), it’s wired directly to the battery (also in trunk) and it does the same thing from there. BB got back to me saying it’s normally 7-9 amps but older units can spike up to 13-14.
I think the load is just too much for the tiny U1R battery. I either need to replace the BB, draw power from the RV hitch plug or see if I can squeeze a higher capacity battery into the trunk.