Redoing Concord Traveler Roof

[Tim Storey]

The camper has a steel roof.  About 25% of the sealing and coating material needs replaced.  There seems to be two or three different materials on there now, and I'd say it was from the manufacturer.  They seem to have used a fairly brittle (at least now) black material as a leveler.  It appears to be only in low spots.  I had to remove some of it where it wasn't bonding due to leak/rust underneath it.  On top of it is white RTV (?) in some places, but usually just as a seam-sealer, then it's all covered with a somewhat flexible material, maybe .020" thick.  I'm trying to get advice from Best Materials, but I'm striking out on the leveler -except they did say not to use "plastic roof cement" in its place.  Any thoughts what I might use for a leveler?  My goal is to do this carefully so it'll last as long as the original, but I do have a budget.  Thanks for your help.