This is a classic low voltage situation. This can be caused by a number of things. The first thing I would do is to disconnect the battery, unplug the A/S from 120V and then proceed to loosen the connections at the Univolt, wiggle them and then retighten. Do the same with the battery connections. Then do same at the 12V
terminal block. If the dim lights still persist, I would have the battery checked as a shorted cell in the battery can pull the whole system down. Given that you indicated that the dim lights occur whether on 120 power or 12V
power would seem to vindicate the Univolt. I really suspect a corroded connection somewhere.