Whats the details on the fridge
@1DayIll ???
I assume the fridge is drawing power from the aux battery and the aux is charged from the vehicle, when stopped the fridge should continue to draw from the aux so the question, what is the charging system from the vehicle ? is it enough to provide charge to the battery and power to the fridge ??? If the fridge is taking more than the vehicle is providing, the aux battery will be drawn down, when stopped the aux battery will not be charged and plugging in solar may not be enough to charge battery and run fridge.......... I would start off getting a load test done on the battery to make sure its okay, a test will show, age of battery means nothing...................... If using Cig sockets get rid of them and use Anderson's............ Aux battery should be getting charged by something like a Ctek 250DS not a simple relay unit from the old days, it will sort out the charge from alternator and cover a solar panel....
If its sitting in an enclosed vehicle like in a ute canopy on a hot day it will heat up and demand a lot of power, inside can get up to over 40 deg, my old engel in Ute would run non stop all day until I ventilated the shed.
I had a small heat exchange fridge to keep my lunch cool in my truck, it only held maybe a 6 pack at best, brilliant when engine running but it would suck the daylights from the batteries if I stopped for a few hours far quicker than a 3 way van fridge and I had 4 batteries in the rack............ so fridge type is important.... Jumper leads off my pan fridge motor to truck batteries would get me going....

... so Boss never knew.