[jimcol]

Quincy, I wouldn't spray the underside. Too much stuff under there that may require access later. As far as on top of the storage compartment many have just bought two-inch insulation and cut it to fit tightly between the framing. Think that would be cheaper than the spray foam though not as airtight. Search the form. There are many threads on what people have done to stop air leaks on their rigs. If you have the return air vent under your steps, sealing that during the summer will stop a huge loss of cold air to the underbelly.