I would plan at least +16' on the RV Width so I couls extend slide outs on both sides if I upgraded to something that had that feature. That would give me approx 4' slideout and 4' walk around space Between the wall and the slideout. Would be plenty to wash with a 6' handle brush if needed.
Drain in floor, 20/30/50 amp pedastal along with water hookup and sewer hookup to dump at home once I backed in. Additional spigot to hook pressure washer or hose to.
Would use a residential furnace to heat. Insulate well and it won't cost you much.