$1800 seems pretty darn high.
It's likely not supported by Jayco because of the framing needed to support an AC unit. If you open the hole, install extra framing, and are able to pull wire (or otherwise run it), you can run it on a separate circuit outside the built-in load center in the trailer. Basically you'd have 2 plugs; the main 30A for the trailer, and a second 20A for that one AC unit. Also, it doesn't need to be another 15K unit. It could be a 13,500 or even 11,000 unit.
Being tied into the current duct work is not necessary, so I wouldn't worry about that. Just close off the vents in the area serviced by the new unit, and force the OEM unit to cover the area NOT covered by the new unit.
My point is that the difficulties Jayco mentions are not insurmountable at all. This can and has been done.
I would be careful with the warranty though, that could be a big problem.
Here's another idea: What about a dehumidifier? I grew up in SE TX, so I have an idea the humidity you're dealing with. Perhaps if you could help the AC unit dry out the air some, that would help?
FWIW, a 2nd AC unit was on the tippy top of the "to-do" list if we had kept our trailer. We had a bigger trailer than yours with a single 15K unit and it couldn't keep up in the summer. I did everything I could think of to eek out every bit of efficiency I could get out of it too. I was fortunate though in that my unit came with 2nd AC prep, so it was a simple matter of cutting the hole. The framing, wiring and duct work were all already there waiting, and we had 50A service, so the electrical system could support it.
-2018 Greyhawk 29MV
-2015 Jeep Grand Cherokee Limited (Primary Toad)
-1994 Jeep Wrangler YJ (Secondary Toad)
-2014 Jay Flight 28BHBE & Ram 2500 6.4L CC 4x4 (sold)