Why complicate things with a "battery to battery charger" (whatever that is)?
Just install a nice heavy wire for charging the camper battery from the truck's alternator. 6ga minimum, preferably 4ga or even 2ga.
Even then it might not keep up if you insist on running the fridge off 12V.
Consider using propane for the fridge full-time. You're not going to blow yourself up or suffocate just from looking at the propane bottle wrong. The fridge does not use enough propane to significantly increase your travel costs, or shorten the length of time you can camp.