Due to the Texas heat I thought about adding a 2nd AC on my Sundance 277RL XLT, but found that the antenna is mounted right behind the vent. Not enough room for the AC to fit without relocating the antenna which would be a bit too much.
So, I checked the ducting to see if there was any reason that the air was not being distributed properly and found that the duct ran at least several feet past the end of the last vent on each end of the trailer. I closed off the space between the last vent on the one in the bedroom, 2 of the vents in the living area, and the one in the bathroom using some cardboard and foil tape. Then I made a baffle to block off some of the extra air that was coming in to the bathroom (for some reason it had a ton of flow). This increased the air flow to all the vents. Also I pulled the inside cover off the ac and checked to be sure it was sealed properly. I found that where the top unit blower slides together with the inside unit, there was no foil tape which was allowing some of the cold air to escape to the outside. I taped that up and anything else that didnt look right and it has made a real difference in how well our unit cools. Its very important to make sure that the blower side and return air is sealed properly, that is one of the biggest areas of loss if its sucking cold air right back into the return air.
In other words, to make a short story long, these mods have made quite an improvement on my rig. Now camping at 97 degrees in the sun, I'm setting in about 75 degrees inside temp instead of 80 or above. We dont camp when the temps reach above 100 unless we have plenty of shade.
Nothing beats a 2nd ac, but there is definately some benefit to be had from checking your factory install and making a few mods.