Safe to leave solar hooked to batteries unattended?

I do it. I have a battery disconnect switch to turn it off going to the trailer (like your friend does) for when mine goes back into the storage lot. I got tired of taking the batteries out and bringing them home to put on the charger. Now I leave them in and with no phantom draws from the trailer they are always topped off. Also like westend said, have a fuse in between there and get a decent solar controller.