What rust proofing is best for 11 yr. old Dodge Ram 2500

That Fluid Film is some good stuff. Last year I did my whole trailer undercarriage with it and spent a month pulling it around Canada in the winter. 8 months later, it's still there and no rust. It's pretty clear that it worked as evident by the areas that were mistakenly untreated. You can either spray it with cans or brush / roll it on with one of the paint cans. Also you can also find a shop that will spray it for you.