Sounds to me like you need a little more info, hopefully I can help.
My father has a venom with three ac's, and this is actually pretty easy to explain why it's happening. Though, it would help if you had a couple more bits of information.
for one, if you're trailer is at 90 degrees with two ac's running, something's wrong, it's 98 today in south texas, and we got theres down to 78. That said, one is on a thermostat, two are knobs on the cooler. The particular one with knobs in the bedroom says it's on, but if you read the manual, it's actually not. Your FAN will run on just 12volts, the compressor will not. Have you actually verified that you're getting cold air out of the actual AC, and the fan isn't just running.
For two, if that is happening, you're likely causing damage, they had to replace the unit because it burnt out the compressor, trying to run 2 ac units on so little power seems to be an expensive problem, at least with the venom they own. I'm not sure how you're turning 15A into enough starting amps, let alone running amps to power nearly 30 amps worth of AC units. If' im not mistaken, two of the three you have are 15k units, they take quite a bit to start, and just a little less to run, that's not including all the others you're using. Though, they don't have solar, so I can't speak to that.
Back to my initial statement though, 90 degrees would be semi expected, if it's actually 110 degrees outside, very few places in the states are actually seeing those temps right now, if you're unlucky enough to get in one, then that's 20 degrees lower than the outside temps. That's about what you can expect, adding another AC won't really change that, these units can only lower the temp by about 20 degrees, it wouldn't matter if you had 100 ac units, if the temperature coming out of them is only 20 degrees lower than the ambient temp, then that's as low as it's going to get, period.
As for why the fans cause it to feel colder, you already nailed that one. At 110 degrees, the static pressure in your cabin is likely extremely high, nauseating almost. It's hard to say for sure, because so few people actually get to expereince such extreme temperatures while in an RV, so what you're seeing is extremely uncommon. Nonetheless though, i think you're right with that one, super hot air is extremely dense, though, depending on the unit, and where the intake is, could stop that. I'm assuming they are all internally inducted, thus using that super hot, likely humid air. I know it's dry outside, but with a 20 degree difference between inside and outside, i'm going to assume your humidity levels are off the charts.
With all of that said, what you're describing isn't all that bad, 20 degrees is decent. I said initially something might be wrong, because it's hard to fathom actual 110 degrees beating down on a big metal box.
Don't forget to take into account the super hot dense air will rise up, so if you're in a small-ish room with a single duct ( like a bathroom ), then you start to push hot air out, you'll feel a noticeable increase in ambient temperature, increase as in, a change, not a literal increase of temperature literally. When you do this on a large scale, the intake can breath more heavily, thus resulting in a higher blower. Unfortunately, with the windows open, you might blow faster, but you likely won't achieve an actual lower temperature, the temperature is static in the sense it's always going to be the same, relative to the ambient temperature outside. so while you can achieve the affects of a cooler trailer, based on logic alone, you can't actually get those, at least nothing that would be worthwhile.
