willieboy

The problem is, if your trailer battery is heavily discharged, it will pull 20-25 amps. I would second the notion to run a seperate 12 volt line for the on-board air compressor. However, you could use the 12 volt charge lead on the trailering harness to activate a relay for the compressor 12 volt feed. Relays draw almost no power.

raisin

What willieboy said for sure. Hard to keep trailer battery charged while traveling. Battery will draw close to max amps at startup. Thats the time compressor will likely run. Use 12v off 7-pin to trigger relay, but run fused separate 12v line to compressor thru relay main contacts. Relay will need 12v from 7 pin to trigger, ground to pick coil grd. Then fused 12v to main contact, other main contact to compressor.