Weekend camping - Fridge use

First take a look at what size battery is actually on your rig (group 24/27/31) and im guessing its a budget group 24. With minimal use for lights/pump/fridge most 12v batts are good for 2-3 days. Throw in furnace and a single night is about all you can expect. Either get a bigger 12v batt, add a matching second 12v batt in parallel, add solar or purchase a generator. Also an idea to ensure the battery that is currently on board gets load tested as perhaps its been drained below safe levels often and cant get the job done.

Best advice I can give that was given to me is google the 12v side of life as theres great advice on the fundamentals of our rv electrical systems.