If today the 13.5 is "almost adequate" the 15 may get you to "barely adequate" that's about it. In our case the 13.5 after the mod's listed below if I set it to 74 on a very hot day it will keep up till early afternoon, then rise to 78 or so before it starts to get ahead again in the evening.
Before going for a bigger AC, take a look at your ducting. do the following
1) take down all the vents then seal any gap between the vent hole and ducting with the metal duct tape, reinstall the vents.
2) add more vents. easy to do, the duct runs are straight, follow them and add more. I added one more in our Bedroom, one in the bath that didn't have one, and two more in the living area. Made a very noticeable difference
3) take down the plenum and again seal up any air leaks between the plenum and ducts.
I think you will find a pretty casual job of the duct installation, fixing it goes a long way to keeping cool.
Before I did the above, the temps would start climbing in late morning and we'd be in the low 80's in the afternoon and after dark before it would start bringing temps down.
IMHO by 25ft trailers may need two AC units. Your battling several issues compared to cooling a sticks and bricks house for heating AND cooling.
1) lots of window area as a % of outside surface, very high heat gain/loss
2) limited insulation
3) large surface area to volume ratio. And it's the surface area radiating heat into the trailer that's hard to cool. cooling/heating air down is easy, cooling/heating a wall is a completely different story.
wanting to get even better I went to tap plastics and had them cut plastic pieces to install on the inside of the non opening window sections to make "poor mans" insulated windows. Then I got perforated shade material that snap attaches on the outside over the windows to plastic snaps using VHB to the side. cuts heat gain but still gives very good visibility.
Now if I set the temp at 74 the trailer will stay that way virtually all day even on very hot days. The AC cycles in the AM then in the afternoon starts running constantly, and I have the thermostat set to go to 70 by 9pm and it doesn't have trouble getting there.
But still, if I don't get the AC running early, it can't bring the temps down much.
Another thing is to do is shut off vents to the bedroom in the daytime, then shut the living room area ones off in the later afternoon and open the bedroom for the night. That helped us a lot until I did the other changes.
