I have chosen (for right or wrong) to do my cavities (on the truck!) and the chassis with Dinitrol. Like you I spent several days reading and reading and ended up going around in circles with all the different advice.
My original plan was to grind all the chassis back to bare metal, use a primer like Bonda with two coats, then a chassis black topcoat, then Dinitrol. However I decided to call Dinitrol and explained that I had a discovery 2 with surface rust, nothing too extensive and is more like rust spots here and there. I explained my intentions and the guy I spoke to felt it would be unnecessary. He said to just brush off the loose rust, jet wash the underneath, leave to dry. Spray any rust with their rust convertor primer then use the cavity wax first on the whole chassis inside and out, then the black wax after.
That is the approach I have decided to take as it sure is a whole lot less effort.