My Outback is 35 feet long with 3 slides. It originally came with a 13,500 AC, only 1 (30 amp). I looked into installing a second one, and the dealer/repair shop I purchased from said "no." Extra wiring, and separate plugs and just basically, a lot of work.
So we decided on at 15,000. Never had problems again!
Couple tips:
If your camper is already "hot" when you turn on the air conditioner, close the slides first, then turn the air on. Smaller spaces will cool faster. After the main area gets cool, then open the slide and the AC will probably keep up .... up to a point.
RV are nothing but ovens! If the outside temps are too much, you can't keep them freezer cold.
Once you get it cool, do not turn the air off. Leave it on, even at night. If the camper starts out cool in the morning, the air conditioner will probably do pretty good at keeping up then.
If this does not help, the 15,000 will make a difference. Still follow this advise about retracting slides first, even with the 15,000 and you'll be fine.
You may have all your holes sealed up, but it's still only got about an inch of insulation between you and the blazing sun. Its not like a stick and brick house you know.