Smallest panel to run a 32L Fridge? Or add another panel to my camper setup/

[the evil twin]

So direct from Aux battery to one of the batteries in the camper will work?

Yep, sure will.

There are a few different options depending on how an individual setup is wired and what solar regs, DC/DC devices are in use etc. but simplest way is just hook the sucker up in parallel.

Keep the lead as short as is convenient say 3 to 4 metres max and use reasonable gauge cable say 12 to 10 gauge and you won't have much voltage drop at all over the extra distance to the vehicle aux.