A 2nd controller would be an option, but quite complicated.
As the 2 batteries are connected at the positive side, they are not really isolated. One can measure the voltage in the car battery quite similar to the caravan battery. The initial point to measure would be the difference between the 2 batteriesm which is the voltage over the 2 MOSFETs. Use a voltage divider and maybe difference amplifier. It may need some care with the protection, because the car system is only 12 V nominal, but can still have quite some voltage spikes from switching large loads and inductive kickback.