I installed a 12 volt manually reset circuit breaker close to my batteries. It is sized to protect wire there and the wire is sized to support my largest draw which is my inverter. It serves as a battery disconnect for storage. This system does not properly protect the wiring harness for the tow vehicle.
Like the post above, I have a self resetting circuit breaker on the "A" frame that protects the trailer part of the tow connection. It is sized to protect the wire in the TT side of the harness.
My tow vehicle (TV) also has a self resetting circuit breaker under the hood. It protects the TV side of the harness and is sized to protect wire in the TV.
Circuit breakers and fuses must always be sized to protect the wire they service. The wire is sized to support the loads. Tables are available that specify the currents different wire sizes are allowed to carry and thus the maximum breaker size that is allowed.
I wish you good luck and happy trails ahead!