For escaping from the escape hatch: Most vent covers I've seen mount with brackets that are screwed to the vertical part of the vent mechanism where it protrudes from the roof. If you find one that fits the escape hatch, you should be able to use wing nuts on the mounting brackets to make them removable from the inside.
I find it hard to imagine a scenario where both the front doors of a class C would be unusable and you'd have to crawl out the roof. In the event of a fire that blocks access from the back to the front, I'd much prefer an escape window to clambering out the roof. Maybe the escape hatches are some relatively recent DOT requirement or something?