Hmmm...We live in a hot and humid locale, so I certainly understand the need for air conditioning of some kind. And you're right, $3k for installation of a rooftop factory type AC is not worth it. Your $300 solution sounds reasonable for limited use, even though the device takes up already limited floor space. Not sure what kind of flooring material you have in the TM, but if it's vinyl or even laminate, the unit's casters might dent the floor after thousand of towing miles.
A cheap and easy solution might be get four of those rubber furniture 'cups' to place under the casters. Or, maybe you could make some sort of rectangular wood frame to raise the casters off the floor and spread the weight over a larger area. If they come off easily, maybe you could simply remove the casters (??)
BTW, we happened to be in BC a few weeks ago - Victoria, and Vancouver (the night of the Stanley Cup playoffs -ugh!!) I liked the cooler temperatures, compared to here
Nick