Being side to side, IMO I think that is a little extreme. You don't want to store your trailer on dirt only not good for the tires and suspension. I would either bring in some 3/4" drainage rock or at least put down some 2x6 treated lumber. You can cut a ramp on one end and stack 3 high to help level it off. His land is probably sloped for water run off, that is why I say 3/4" rock. For one thing you have a good friend to allow you to do that....