There is no issue with getting on the roof of any RV...don't know where you heard a blanket statement like that. All RVs are meant to have their roofing cleaned and access for other maintenance (antenna needs lubrication to keep it from leaking - don't miss that one!, A/C covers need to be removed and the internals cleaned, etc.). The fact that there is no ladder just means that you shouldn't be going up there unless you have a good reason to. The roof is not a storage area (don't even think about putting holes in the roof for a luggage rack) or a seating area at the race track and many manufacturers don't want to spend the money to put the backing (frame supports) in to support a ladder and the expense of the ladder itself.
If you weigh on the "heavy" side, you might consider putting down some plywood to distribute your weight, but, being on the roof is not an issue.
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)