I wouldn’t recommend installing a swamp cooler.
The 2004 921’s should all be prewired for combination heat/AC thermostat, Roof AC, Fantastic fan, Microwave, stereo and speakers, and TV antenna whether those options were ordered or not.
Any standard non-ducted RV roof air conditioner can be installed in the roof vent opening that’s in front of the refrigerator. There should be a blank electrical box cover in the ceiling next to the vent opening, and that’s where the 115v and thermostat wires will be stashed. There may not be a breaker in the power center for the air conditioner, but the wires will definitely be there. You may also have to install a new combination thermostat.
This is what the electrical box in the ceiling looks like in my camper.

This is a screenshot from the 2004 Lance brochures the 921 interior, and I’ve circled what I believe is the air conditioner prewiring box in the ceiling.

:):)