To move around inside the vehicle I’d expect you to move on seats, cargo, aisles, doors (including trunk doors), or tables (I may very well miss something, as it’s from memory). Everything else would be a matter of climbing around (typically on the frame).
I haven’t encountered your situation, though, as I’ve use all the interior space for something (although I’ve considered whether to replace the aisle with cargo, but so far I’ve decide that the carrying capacity is sufficient, and it’s probably not worth slower interior movement).
My essentially stationary craftmobile has some exposed frames with a canvas ceiling, but I’d expect canvas to be unable to support a character, and my character remains inside. Apart from time spent moving, I suspect it doesn’t matter if you’re on the roof or inside when moving about, though.