If manufacture says it's walkable, then you can walk on it. My unit does not have a ladder (due to a rear slide out king bed), but manufacturer brochure states it is a fully walkable roof.
SAFETY FIRST at ALL times! Only do this if you are agile, feel comfortable doing this, and have assistance. Do NOT do this while alone!
I'm 6' 275# and I am VERY CAUTIOUS when I go on roof. In the next week or so, I plan on removing AC shroud to clean evap and condenser coils, and give it a general inspection. My 23yo son will assist.
Ladder will be placed so when I get to roof top, I'll be near AC. I'll go straight to AC and stay near it for my work. I will use a 2'x2' piece of rigid foam to kneel on (easy on knees and spreads load). Plywood has sharp edges, is heavier, and not needed IMO. Rigid foam is plenty rigid for this application.
Except for one fantastic fan vent in center (which I recently had installed by RV shop) I was able to caulk all other areas. If I can do any roof-related work , i.e., antenna repair, vents, vent covers, caulking, etc., from side of RV using a ladder, I'll do that. I'll ONLY go on roof when I feel safe and want to do work myself.
I like to use a straight folding type aluminum ladder, with pool noodle foam taped to upper sections to protect where ladder touches the trailer. I prefer this type of ladder over the A frame as I believe it is more stable when stepping to/from ladder, onto/off of the RV roof.
Go slow, take your time, don't rush, stay away from roof edges, and have someone with you to assist with handing you tools/supplies, and to hold ladder when you go up/down.
Again, SAFETY FIRST!!!