Earlier this winter I was ready to buy a 12K Everest but the supplier raised his price about $100 dollars so I put the modification on hold. I did lot of research and my choice of furnace
for my 13 Scamp was also the 12K Everest mainly for the power draw specs.
I know you wanted to talk to someone with a "Been there done that, T-shirt" but I have only been there and I haven't done it yet. I just thought I would let you know what my thought were since it looks like I stopped the project at about the same point you are currently at.
Another problem with a closet mount besides depth is the gas line reroute. Under the sink you should already have a gas line convenient to hook into. The depth of the sink cabinet to the outside wall should work with the standard exhaust port. ( About 20"). The sink cabinet cutout should be just slightly larger than the furnace. Since you previously had a furnace you may have to make an aluminum trim plate to close up the old hole and cut a new hole for the new furnace. You need to mount the furnace high enough to clear the wheel well. I was going to make a rear furnace support to mount to the wheel well. Scamp mounts the thermostat to the sink base cabinet (Not desirable.). I was going to mount the thermostat to a mounting plate on the spiral supports or on the end of the upper cabinet if I replaced the light
with a led fixture. I also was going to run the thermostat wire up the spiral support buy using shrink tubing or up the wall. You may also need to fabricate a plate for the outside of the trailer for the intake/exhaust port.
Hope this helps.
I saw your slide show after posting this post. Maybe you can move your converter to the lower front LH side of the sink base under the furnace and in front of the wheel well. Looks like things may fit but it will be a tight fit.
Great job, and good luck