Hello helnick

May I be one of the first to welcome you to the forum

I am not an Electrician

You have asked a very generalised question, and I shall give a very none technical answer, in layman speak

It depends how your 12 volt system is set up

Scenario #1
You have a CTEK D250S Duel battery management system (12 Volt DC to DC charger) at your house battery.
This system operates by having both the Car Engine Battery, and the Roof Solar panel, connected to the house (caravan) battery.
When the house battery is fully charged, the roof solar will then start charging your car battery, when the engine is switched off
In this scenario they YES you can charge another 12 volt battery, through the uncoupled Anderson plug

Scenario #2
You have some sort of system which disconnects between the car and house battery, when the engine is not running, or the ignition key is switched off.
The leads are connected to a battery management system, same as above, but of unknown manufacturer to me
In this scenario there are just too many unknown variables that I cannot answer yes or no

Scenario #3
If the Anderson plug leads are connected direct to the house battery, then if the spare battery you wish to charge is of the same chemistry/capacity/age, of the house battery, then YES you will charge or at least equalise both the spare and house battery

But...
If the spare battery, and the house battery are not the same chemistry/capacity/age, there is an excellent chance that you will be overcharging one, and undercharging the other battery.
Therefore NO do not do it, as you may permanently damage one battery

Hope that this makes sense to you, as it was the best way I could describe it, instead of guessing a yes or no

If you put up in the techies part of the forum, the exact 12 volt system you have, then I am sure that you shall receive answers from folks, who have much more knowledge than me

I hope that you have many happy road trips ahead of you

Rooftop. Tent. Tinny. ?????
You want battery mobile or fixed. and what for..