Need Advice on Insulating Under the Trailer

I Spray Foamed mine! You can get a DIY kit at those same box stores as OB recommended. There is about 600bdft in the kit which should yield you about 2" on the length of your rig. Or look up a local insulation contractor. Just make sure to wrap the tank valves and sensors in fiberglass batts before application. Once SF is applied it is almost impossible to remove. The FG makes it easier to break loose in case a repair is needed.
Here is a link to a post with pics on what I did.
Mine came from the factory with Coraplast and I would recommend that you still install it even if you foam it.