Hello Matt -- --
Here's one approach you might consider -- in the trucking industry to combat ideling laws -- there are several companys making diesel generators and electric HVAC units.
Here's a link to generators when I was researching:
http://community.webshots.com/album/47421720vXokAd
I selected this unit for my application:
http://www.truckgen.com/
Considering the size of your box -- you may need 2 HVAC units.
Here's a link to the install of mine -- although I've not taken pictures of the under bunk HVAC unit yet:
http://community.webshots.com/album/68827129iOaMmN
Gives you something to think about -- --
I'm off to the diesel truck races today at Woodburn Drag Strip ........... :^)
OLDphoneman