The coleman roof mount air units are massive in general, the biggest size and weight wise in the RV market, even the one you linked is 8 lbs heavier than the carrier that you linked earlier.
The way I see it you have two problems with putting an RV air unit on a truck cap. The first is power consupmtion, and the second is structural.
Power wise, the on-board inverter is rated to 400 watts at idle in neutral and or park, and 100 watts in gear. Even if it were rated at 400 watts while rolling down the road you are well short of your power requirements. I am no electrician, so on this second part I may be wrong, but even if you were to put a 5000 watt inverter in the truck, and upgrade all the wiring to be able to withstand that amount of juice, I suspect that you would be sitting on the side of the road with a dead battery and or fried alternator in no time at all, since I am pretty sure that even the bigger alternator that comes with the towing package would not be able to keep up with that load as well as the rest of the electrical demands of the truck. The only way to run an AC unit like these is off of shore power, or a generator.
The structural problem comes in to play because RV air units are not solid mounted to the roof, they are mounted on a very tough but flexible gasket. This gasket acts like an innertube, producing an air and water tite seal, but at the same time permits the air unit to bounce a bit while running down the road. not to mention the pulling and pushing fprces that will be exerted on it from the slipstream of air that hits it as you are driving, or the crosswinds. Also while a cap may be rated for a roof load of say 100 lbs, that would be 100 lbs evenly spread over the entire roughly 4 foot by 6 foot cap surface. This would be nearly all of that load in a 16 x 22 INCH
section of the fiberglass.
Give the donut idea a try first. If you wanted to try it out before spending the money on the ebay
item I linked, go get a 26x2.2" mountain bike innertube from your local bike shop (should be about 5 bucks) and put it in place and inflate it. Then turn the air on max and go for a drive and see what happenes. That coupled with limo tinting the cap windows may be all you need. Afterall, the Tacoma shares a platform and a good bit of parts with the 4-runner, including AC compressor. On a 4 runner it manages to keep that much area cool. If that doesn't work out, then I think you need to perhaps re-visit the swamp cooler idea, or keep looking for a much
smaller AC unit. Example, my RV has a 15,000 BTU air unit on it, and it is 21 feet long by 7 1/2 feet wide, by 8 1/2 feet tall. (1338.75 cubic feet) That air unit is capable of making it downright frigid in the trailer even at mid-day in the sun. You are looking to cool an area that is 6 feet long, by 4 feet wide, by 4 feet tall (roughly) (96 cubic feet), which is significantly less.
Sorry if it seems like I am pissing on your parade, I am just trying to help.