The 13.5 A/C will not cool rear bedroom on DP. What to do?

Look at adding some good double foil quilted insulation under the bed over the engine and around the sides of the bed box.
I did that and it made a fantastic difference.

I do run the generator and both A/C'S while traveling.
2002 Fleetwood Discovery with the air ducts in the roof.
One small vent in the bedroom roof. I wish it had more air
volume blowing through it.
Maybe the 14 year old A/C's need updated.

Need a lot more info. Year/Brand/Model of motorhome. Ducted or NON ducted. Your problem is the opposite of what most RVer's have. Due to the Bedroom having to turn over less volume of hot air, the bedroom AC usually makes it quite cold in the bedroom. Are you positive that the rear AC is cooling at all? going from 13.5 to 15 will not create a greater cooling factor. Will help a little but if NOT hardly cooling with a 13.5, then the extra 1500 will not help that much more for the cost.

Park in the shade.
Run both roof AC's using the generator while driving.
At night run the bedroom AC to a very low setting. Then during the day it'll have a chance to keep up.