go look at http://www.airforums.com/forum...ign-23313.html
and you can see what it looks like once you take the cabinet apart (my cabinet is remaining apart--new birch plywood on its way).
You will have to remove some rivets on the skirt that is to the outside (toward the curb, oh my gosh, your bath is on the curb side, isn't it? if not, reverse my directions, as required) of the toilet and under the shelf under the basin. Take that skirt out. Undo the two compression fittings. Go outside and take out the 12 screws around the perimeter of the water heater. Undo the propane line and either slide it down into the floor (from the outside) or cut off the flare nut as close to the nut as possible then slide it down.
The water heater should now pull cleanly out of the opening, with only a little sticky from the plumber's putty around the opening. Putting a new one back is a 1-hr job.
I recommend you paint the floor with an oil/alkyd based paint before you put a new water heater back in.