Its been awhile but I think this would be your basic relay setup to isolate during operation. Ground #85, get 12v from the winch motor for #86 when the winch motor sends power it will activate the relay and seperate #30 from the normally closed #87a. So then all you need is your ignition on #30 and #87a will be your output to activate the solenoid and charge your battery. Someone can double check my setup there may be alternate ways to do it but I think this should do the trick.
I should add that this may get you by with a smaller gauge wire but I have to agree with RioGrande adding heavier wire or a true dual battery setup with heavier wire is a better option IMHO.