Yes - we retrofitted a big-ass leisure battery to our last caravan. It lived in the wardrobe, in a plastic bread bin (to deal with spill of acid if that were to happen).
We tweaked the car - ran a 20A rated big fat wire to the alternator main bolt (with a 20A local fuse there). In the boot, we put that through a Schottky diode then onto the 12N socket, plus a 3pin metal socket in the boot[1].
The diode prevent the caravan battery trying to back feed into the car (ditto point [1] below).
In the caravan, we took the local battery and the car supply from the trailer plug to a changeover switch on a plastic box on the wardrobe that I made. This had a voltage sensor (741 op amp) and a relay and if the car voltage dropped to about 11.5V it cut power so the car had enough to start. Then we flicked the switch to the local battery. That way, the local battery did us for 2 weeks with the car supplying most of the power and the battery just making up any shortfall.
That was running several fluorescent lights, water pump, and a tv.
I did try to make a voltage booster so the car could pump some charge into the caravan battery whilst parked, but I couldn't make it work.
[1] My mum was disabled (stroke left her weak and unable to walk long distances) so she had an electric scooter. That went in the boot and plugged into the 3 pin socket and charged in the same way as the caravan, except it got charged on every day out - which surprisingly, seemed sufficient to keep it charged. So the diode again prevented the gel batteries from blowing up trying to start the car.At 0.6V forward drop, it did not seem to impeded the ability of the
14.4V on the alternator from charging stuff.