It will work, I actually built a demonstration unit last summer here in southern california due to the hot weather. The results were good but the only drawback is it takes up alot of space inside my van.
If used inside a vehicule it needs to be built more sturdy and I would not use the blue durakool evaporator pad, it clogs up after several months and restricts airflow. One more thing, he got cool temp outputs because the outside air going into the evaporator was cooler, in a vehicule because of the heat of the vans metal body, the air temp will be higher, water temp will also be higher.
Right now I'm still using my swampcooler, water usage hasnt been bad, 1 gallon total water usage over a 2 day weekend.
I'm building a smaller all in one unit indirect/swamp cooler, when completed I'll post pictures and temp results. If I can get 2 or 3 degrees lower air temperature it will be worth it. On a hot day you can feel it.
Heres a picture of the indirect cooler (heat exchanger) I built last year, this was just a test, but notice all the space it takes up, the actual evaporator pad was about 5 feet away and it also took up more space.
In my design I use the heat exchanger to cool the outside air which was 115f, which got cool down to about 102f, then that air went to the evaporater pad which cooled it further to about 90f, the water temp remained a constant 86f. Those are real life temps you might see in a hot vehicule on a very hot day. 90f seems high but on a hot day it feels good.