Barb complained about cold feet in the version 1 vehicle; not surprising since the school bus wasn't designed for extended occupancy in cold weather. The manufacturer of a popular line of mobile grooming vehicles acknowledged this problem as well. The new body has polyurethane insulation all around, including four inches in the floor. Even the stairwell is insulated. On top of that, there is an LP powered furnace, the Atwood 8520-IV, a 20,000 BTU unit. That should make things toasty. Residential furnaces start at 25,000 BTU for a small house.
The furnace was one of the most difficult design issues, since it had very specific requirements for airflow. These are discussed in the Design portion of this site. The biggest part of this installation was cutting the hole through the body. It was difficult mechanically and psychologically. The location of the furnace changed a bit once construction layout began. It became clear there was no return air intake on the furnace. Essentially the outside of the furnace was the return area. If ducted return air was required, the entire furnace would need to be enclosed in a cabinet. So the equipment space became a return air plenum.
The other major requirement for the furnace was to have four output ducts to assure adequate airflow through the unit. To accommodate this, the unit was moved as far as possible to the rear of the equipment space. This required cutting through the steel that makes up the vertical corner of the body.
The opening was cut first from the inside, cutting through the exterior, using a reciprocating saw. Drift between the inside and outside surfaces was not a problem. The outside opening would be cut larger to accommodate the recessed door flange. In the photos below, the first, rough cut, shows the first cut-through. The second shows the opening cut to receive the mounting flange.
Below are some factory drawings of the unit, and the wiring diagram.
Installation recommendations included the requirement to use at least four of the available outlet openings on the furnace. These are routed with flexible ducting to a manufactured plenum, shown below. The round ports connect to the flex-duct from the furnace. The rectangular opening ports through the wall into the grooming area, and into a box that supports the bath tub and allows connection of a small opening into the bathroom.