I have a Jayco 28RLS with one 13.5K ducted unit. On 90+ degree hot and humid days the unit has had no problem keeping things cool to my wife's satisfaction (meaning quite cool). We start running the unit in the morning so it never allows the trailer to get really hot during the day and then having to play catch up.
As others have mentioned if your unit is already wired for 50 amp adding a second A/C isn't too expensive (price of A/C and basic installation). If your trailer is wired for 30 amp then you could be talking $2,000 for all the wiring, new converter and A/C unit.