I have never heard of a motorhome where you can't go on the roof but, if your manual says not to do it, I'd call the company for clarification. I do know there is a brand of travel trailer where they do not want anyone on the roof. Won't support the weight.
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.
Cleaning can be done from either ladders or scaffolds set up NEXT TO the motor home, Note, use all proper ladder/scaffold safety precautions.
Repairs..... Well, if you can't go on it then scaffolding OVER the roof is the only option. As someone who was once a professional roofer I know how to do this (it's easy) but I'd not recommend it for those who are timid around such as ladders (Note: This includes ME, Roofing is a strange person for someone acrophobic, but I did it anyway)
In some cases you can "Plank" the roof (lay 2x8 planks where you are going to walk) however without more information on the construction of your MH I'd not suggest that