A lot of Travel Trailers have what are called non-walkable roofs. This is due to the way the roof is constructed to save weight. In my last TT, the truss members were just thin pieces of metal that didn't look like they would hold much weight. If you are on the lighter side (maybe sub 200), you could probably walk around on the roof without an issue as long as you try and spread your weight across trusses. This is the method I used on my TT and didn't run into an issue. If you need more support under your feet, I have heard of people using some thin wood to spread their weight more evenly across the roof.
Your main concern would be bending a roof truss, which could change the pitch of the roof and cause water to pool in the wrong places. You also don't want the roof flexing too much and potentially tearing the sealant away from the seams. Or worst case....falling through the roof!
