Normally with 50amp service, which is 220v service, half of the 50amp 220v breaker feeds the entire TT as 50amp 110v and the other half has nothing wired to it because it's reserved for the second AC unit (if the trailer has a washer and dryer then they would also feed from this side except that the dryer would feed from both sides because it needs 220v), so there should be no breakers installed for the second AC side (normally there would be a 20amp breaker which feeds from the 2nd AC side of the 50amp main breaker) It's from this 20amp breaker that #12 wire is run to the 2nd AC unit. I don't know how or if you can conceal the wire somehow in the walls or by going under the TT with wire loom and up to the converter and up to the ac unit), this leaves us with “is the hole big enough to fit the AC unit" question? Here's a manual for the typical Dometic 13.5K unit which has the dimensions for the hole in the roof: http://eaa1358.com/Passport/Air%20Co...20AC%20(2).pdf
Looks like it's designed to fit into a standard 14 X 14 inch vent hole, now the only question is "is the hole reinforced enough to carry the load of the unit?" I would ask Keystone if they put enough reinforcement at that point to install the second AC unit there when the trailer was manufactured. My best guess is that they did.