I know eggcamper
has double wall, with some insulation. Think Uhaul
is also double wall from posts on making attachments to walls.
Considering the total size of a 13 ft or even a 16 ft heating for human comfort should not be a problem spring and fall
. With just ensolite or carpet lined walls for insulation. Water and waste holding tanks freezing as I think Jim alludes to could be a problem if temps are low enough for long enough.
Fresh water is inside but generally not in heated space, like under a seat. Not as cold as outside but not directly heated. Waste tanks are outside, under trailer so those need the electric heating pads or non-toxic holding tank anti-freeze added if you expect temps to go below freezing.
If you have a shower you may find that the drain pipe P trap (side ways S shaped bend) is outside, you would have to pour a little of the holding tank anti-freeze down the shower drain so that bend is filled with anti-freeze not water, replace anti-freeze after using shower. Should only take about a cup.
Size of tanks and how full they get will determine how much waste tank anti-freeze you need. Week end use that only half fills tanks will require less than if you will be using it long enough to fill holding tank. So you may need to add anti-freeze (dumping down toilet and shower drain, or sink) as your tanks fill up.