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From what I've gathered, a rule of thumb is that a space 35'x8'x8' needs between 5000 and 10000 BTU's to heat or cool depending on average outside temperature and quality of insulation.
Tinted windows reflect radiated heat and work well in Colorado, so if you are traveling in hot, sunny climes I highly reccomend it.
If you get 2 5000 BTU units, you can run both or just one and have the ability to cool rapidly (both) and then keep cool efficiently (single) depending on airflow inside your rig and the outside temp. Since the interior of a rig is smaller than 35x8x8, you have some room to "Fudge" and may be able to get away with 2 smaller units.
You can also purchase Carrier overhead units, rated at 20,000 btu or less that we mount on actual school buses here at the dealership I work at, and they are relative cheap and work great if you have a decent generator.