Building a Heavy Duty Charging Circuit for Camper--how big?

You do not need a relay to control the relay/solenoid. They draw 1-1.5 amps and you should be able to tap the radio fuse circuit to drive it.

A wire about the size of a pencil will work with a 40-50 amp thermal breaker versus a fuse. Be sure you match the ground wire size too. You can google 12 volt thermal breaker. They are available at an automotive store. They will trip and automatically reset if too much current passes. Easier than chasing blown fuses.