Spray Foam Undercoating / Insulation

Just remember that to be effective, insulation must be continuous and that means if you are insulating the underbelly that the sides of the I-beams must be covered as well. Then if you only add insulation to the underbelly between the I-beams, then a full 25% of the floor area (I-beam to wall) receives no benefit. And that is not counting the slide out floors which are the least insulated. Finally, there are the slide out mechanisms that can't be sealed with spray foam.

Skirting using rigid foam board may be more effective if the unit is stationary.