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

#8 is fine, I would go up to 50 amp fuse.

Use a bidirectional relay to control power. When voltage on either side indicates charging the relay closes. This will send solar power up to keep the start battery charged and when the alternator is running send power back to the TC house battery.

Try cleaning the fridge burner. Also add some screening or home furnace filter media to reduce the wind effect.