I don't think Champion, the manufacturer of Ultrastars, is around any longer, but there is an owners group web site at http://www.ultrastar.org/
I don't know if the "don't go on the roof" statement is a legal liability precaution (RV roofs ARE dangerous places) or a warning of structural weakness. If it is structural and the roof is flat enough, you may be able to perform any necessary maintenance by putting a square of plywood on the roof to spread out the weight load. A 3x4 foot piece of 1/2 inch plywood should be sufficient. If it is as curved as the pictures suggest, plywood may not work very well. You could try a narrower piece (e.g. 2x4) and run it fore & aft so it doesn't bridge the curve too much.