check your fuse panel to see if you have a breaker labeled for a second ac. If you do the wire is beside the vent in the other room. If you don't see a electrical box take a screwdriver and remove the trim ring around the vent in the inside. The wire might be in a box beside the vent inside the ceiling. Once you find the wire adding the ac is easy just remove the vent and follow the instructions with the new ac. It mounts using the same hole the the vent was in.
If you don't have a breaker in your panel for a second AC then it is likely that your not prewired. If your not prewired it would make a 2nd AC installation much more complicated. In that case you may consider a Fantastic Fan ...
Clink on this link and then scroll down till you get to my second AC install. It should give you some ideas of what's involved..
I cheat.... Our trailer is 30 AMP & I can run two ACs at once........ but this will not work everywhere nor for everyone........ My RV dealer / owner & mechanic suggested this.... two services. One is the 30AMP main trailer..... The other is a separate breakered fuse box that I run off the 50 AMP pedestal plug......... So I have two main power cords coming off the trailer. The secondary is only for the second AC and we have a second electric frig for drinks etc........ & anything else in the toy room. (ran separate outlets in the rear room)