.3 volts is probably just transient voltage on the line. Use a test light to check for enough current at the fuses.
Are you sure the Unipower is actually getting 120 vac? Check at the Unipower terminals. Is there a fuse inside the Unipower?
Is there 12 vdc at the battery terminal? The battery charger should have an independent output, seperate from the other 12 volt
Check to make sure the Unipower has a good 12 volt
ground. I think mine has a ground at the front of the trailer, where the trailer plug enters the trailer.
Another quick test is to use a 12 volt
power supply to connect to the filed devices. I use a cheap battery charger from Harbor Freight, which has a 2/6 amp output. It's prtected against shorts, and puts out enough power at 2 amps to operate lights, etc. Pull the fuses at the fuse bank, and connect to the field devices. Make sure you connect to a frame ground, not the battery terminal (It might not have a good connection).
If you get the filed devices to operate, then the Unipower isn't putting out voltage.
In short (pardon the pun), find out if the Unipower isn't working, or if the filed devices or wiring is bad.
That's a start!