What would you do?
He IS a bastard, but she still needs the driveway cleared so she can drive. I would leave it there, and make excuses about your trailer, etc., unless he going to set it outside, or damage it or something. You always going to take care of your MIL, regardless, well, this is that "regardless" moment. He's a POS, but if he's not going to sell your stuff, then leave it there and tell him your doing it for his wife, and your wife's mother.
My Bro-in-law and mother in law lived in the same town as us for years. MIL go divorced, so she asked her son to help out regularly.This one time she needed help, I was at my bro in laws house finishing up an addition I put on their house (for free I might add, well Beer..:)), and he asked me to go to his moms to take of her! guess what, since she needed the help, I did it anyway.
Point being, she needs your help, and it's not her fault her her husband is a waste of space. Good luck.