How to isolate charge circuit from drain circuit
What you really want is a current limiter on your charge circuit which it may already have. Check the specs on it.

For a more manual solution you can do this with an double pole switch to the winch (you may want a switch anyway). In the ON position it connects the battery to the winch but in the off position it connects the battery to the charging circuit.







simulate this circuit &ndash; Schematic created using CircuitLab

Add a interlock of some kind so you are encouraged to leave the switch in the OFF position (like for example using a key switch with the key on the same ring as the one for the vehicle doing the towing).