From what you describe, there could be 3 problems. The converter/charger isn't charging or has a bad fuse, the new battery got too discharged, and went bad (again), or there is a current drain that exceeds the charger's capacity.
My suggestion for testing would be:
First, make sure ALL the screw connctions are tight, and turn off all lights, fans, furnaces, fridge, etc. (You don't want something on, while testing). Make sure you know how to safely test electrical circuits!!!!!
2nd- disconnect all the PO fuse holders they added. You don't know what they go to yet, and it's not part of the charging circuit. You can reconnect them later. Make sure all the fuses are the right rating. Was that a picture of a blade fuse in the fuse holder? REPLACE it with the proper SLOW-BLOW fuse (30 or 40 AMps, probably) A fast blow automotive fuse won't work. If that's the charging cisruit, that's the bad fuse!! (Just make sure you're starting out with all the wiring connections and fuses in good order.)
3rd- with a VOLT (not OHM) meter in the 12 volt
DC range, test across each and every fuse. If the fuses are good, you will get ZERO volts. If a fuse is blown, you'll see 12 volts across the fuse if there's a load still connected somewhere.
4th- test from each fuse, both sides, to negative. You should see positive 12 volts at every fuse (I don't think there are any switched circuits on your unit...anyone?). If you see 12 volts on ONE side, but NOT the other, the fuse is blown.
5th- With AC still on, test across the battery terminals. Should be about 13-1/2 volts. Your charger is working properly.
6th- With AC off, test across the battery terminals. Should be above 12 volts. Your battery is propbably holding a charge. Test this again in an hour, see if the voltage drops. If it drops, the battery's weak. If not, it's OK.
7th- If steps 5 & 6 check out OK, then disconnect the converter (I assume you still have the old Univolt?) from the 12 volt
dc fuse block. Place the volt meter in series with the battery to the positive terminal. (Remove the battery fuse and connect the meter) You shouldn't get any volt above 1 volt. If you do, re-test with a test light. If the test light turns on, you have something connected that's draining the battery or there's a short in some wiring.
I suspect the fridge drained the battery last year, and when you put in the new battery, the automotive fuse in the charging circuit blew. Charge the battery with a battery charger outside of the AS, replace the fuse with proper rating fuse, and you probably will be fine.
An electrical supply house, or Inland RV will ahve the right fuse. Radio Shack probably doesn't carry the right fuse. (DON'T bypass the fuse 'cuz you don't have one!)
Then, let us know what you find.