So that would suggest
@terry_evian can make a good start with a fairly typical jayco setup? A deep cycle battery connected to the existing setec battery terminals, a solar panel (portable or fixed) connected to a solar controller which is connected in turn to the battery, a wire from the setec to the 12 pin plug that allows the car to also charge the battery.
Then a basic dual battery setup in the tug to run a second fridge.
2 batteries, a solar panel, a mppt controller, a basic dc dc charger, some cable and connectors should squeak in under $1000?