I think the 15K AC is a great upgrade, especially in warmer or more humid climates. Ours works great in a 31' long with a large slide out... but remember that AC's can really only produce about 20-30 degrees colder air than outside air. If its 110 outside, a properly working AC unit might only make it 80 degrees inside...
If your unit is icing up, even on high setting, something may be wrong with your unit. I would suggest you get it looked at. It could be dirty and restricting airflow, it could have a problem with the capacitor or even be low on refrigerant.
If you are trying to save some money, check into those first. If you want a new AC unit with a lower profile, I say GO FOR IT! We camp with friends with a smaller trailer and a 13.5K unit and they struggle to maintain comfy temps. Our bigger camper with a slide and 15K cools faster and more consistently.