Want to make a Gen slide - how thick of Alum plate?

Aluminum isn't bad by any means, it's just soft. You may need to reinforce it underneath with some angle iron to keep it from bowing even if most of the weight is on the sides where the feet are. Considering a MH doesn't just sit there, it bounces down the road, and those roads aren't getting any better, I'd say you will need a piece of 1/2" to keep it from bowing. A piece of 3/4" plywood and some paint would be a lot cheaper and look good also.

Bill