It's not a bad idea if you will be spending a lot of time in very dry warm places. We have a house sized swamp cooler on our house in Phoenix, and can make it into mid/late June before having to switch to AC. That's 110 outside, usually 80 or under inside. This requires single digit humidity. Toward the end of June when the monsoon season starts and humidity gets into the teens and 20s it's all over. Inside will be 80s to 90 and Florida sticky. The easiest way to tell is watch the dew point. Dew point above 50 degrees is where it falls apart. You're probably already aware, but to work an evaporative cooler must draw in warm dry air from outside. Cool Damp air must vent out through open windows or vents. You can't recirculate cooled air. You can (and we do) try to "super cool" down as low as you can overnight and then seal everything up tight, and do everything you can to keep insolation down and "coast" as long as you can. Otherwise the cooler has to run continuously, and while less than an AC is still not an insignificant amount of energy consumption.
In terms of the specific design I think you are significantly underestimating how much airflow, water, and evaporation area you need. A bus is not a huge amount of square feet, but like a car suffers significant heat load from window area and insolation. Despite the small volume, many cars/trucks/vans/SUVs require several tons of AC cooling power. What would normally be enough to cool a house. The bus might be a bit better if insulated and you remove or block out windows, and will be better if parked out of the sun and at night. I'd still not be surprised if you need 2-3x the cooling power of the equivalent residential space. Also moving air through the evaporating medium is much more difficult than just moving air round in the open. A little 120cfm vent fan probably isn't going to do much of anything. My house unit uses a 1HP dual speed motor on a giant squirrel cage fan to pull air in through the pad. My portable in the garage is small but still has a substantial squirrel cage blower.
I would look at available portable/window units to get some thoughts on sizing. Here's one example of a smallish window unit, $354 from Home Depot:
Champion Cooler 2800 CFM 2-Speed Window Evaporative Cooler for 600 sq. ft. (with Motor)-WCM28 - The Home Depot
Stated to cool 400-600 sqft, so probably about right for a bus.
- 2800 cfm squirrel cage fan
- Not a standard size motor, but rated 4A @115V =460W
- Intake / Pad Area: 2x21"x13"+1x21x20 = 966 sqin = 6.7 sqft
- No spec on water flow or usage, but a universal cooler pump rated for up to 5500 cfm is spec'd at 245gph.
Both my house and portable unit use a constant water supply, so I don't really know how much they use. But reading around a little I wouldn't be surprised if the unit above was approaching a gallon per hour. Remember it's only the water that evaporates that gives you cooling, and it takes a little over 8000 BTUs of heat to evaporate a gallon. So that gives you some idea of "cooling power" vs. water usage.
Also in the manual for my home unit it states that you need roughly 2sqft of exhaust area (open window, vents, etc) per 1000 cfm.
Rob