Make sure you can actually add it first. We wanted to add a second one to our previous travel trailer and was more than willing for our dealer service shop to do the work. When consulting with them, they said they wouldn't do it.
First, the vent over the bedroom was not wired. I suggested just putting in the air conditioner and run a separate wire so I could plug it into a normal 15-20 amp outlet. I'd run an extension directly to the campground electric hook up. They wouldn't do it.
Second, I asked them about adding the wiring inside the camper, and they reminded me the camper was a 30 amp system and the entire camper would have to be rewired for 50 amp. We're talking big money there. I didn't want to do that.
We ended up switching the 13,500 btu for a 15,000 and stayed with just one air conditioner and it actually worked pretty good for a 35 foot TT. But... we camp in Florida in the month of December too! In the Northern states, it's very seldom THAT hot.
It turned out the single 15,000 BTU was the best solution for us in the long-run.
So, just an FYI... make sure, before you go out and get all the hardware and spend a bundle on a second air conditioner, it's actually do-able on your camper.
Of course, if you do the work yourself, everything is possible! Dealerships and service centers have to follow rules and legal guidelines and also have legal obligations and can face law suits if something went wrong down the road. So, in order to avoid things like that, I think it's just easier for them to say, "No, it can't be done."
Edit:
I still have that 13,500 btu air conditioner sitting in my utility shed at home. I'm still trying to give it away to anyone who wants it ... been making this offer for 5 years now. Hope the mice haven't chewed up all the wires now.