If you only have the one way out at the rear, you may want to consider an egress window (that pops out for an emergency exit) or a roof escape hatch. Both are available on ebay or the surplus dealers in Elkhart. If you are worried about not getting to that rear extinguisher, maybe you should also worry about not getting out the rear at all. That sort of thing is requirement on any RV, and certainly not a bad idea on a homebuild. I have a 22x22 roof hatch ready to install over the bunk area in our gooseneck trailer where the bed is. Installs just like any other roof vent, but larger and set up to open up completely to get out.