First off, the magic cure for every air conditioner that is not cooling properly is not to put freon in it. There are several things that can cause one not to cool. A loss of refrigerant is certainly one of them, but these units only take 18-24 oz of refrigerant, so a leak is likely to completely lose the entire charge.
These units can be checked for proper charge using a clamp-on ammeter. The current draw of the compressor indicated on the manufacturer's plate is based on an outside ambient temp of 95 degrees. Every 10 degree change in ambient temperature will cause a one amp change in current draw, an increase of one amp for a 10 degree increase in temp and a decrease of one amp for a 10 degree drop in ambient temp. This will tell if the unit is charged correctly without having to tap into it with a piercing valve and attach gauges.
Another visual indication is, after running a while under load, the suction line should be sweating(cold and wet) back to the compressor. The suction line is always the larger of the two. If it is not sweating, then you are probably low on charge.
If you have a leak, it will do little good to sweat in a service port and recharge it without fixing the leak. A reputable tech should be able to do this and recharge in a couple of yours. Figuring the current labor rate of most shops at $100/hr plus freon, you are probably looking at $250-300 for a recharge.
Personally if a nine year old unit can not be made to perform by cleaning the return filter and the coils, I would replace it, especially if the repair bill is approaching half of the cost of a new one.
One more thing.. Size matters. I do not understand why Coleman and Dometic even bother to build 13.5K BTU air conditioners. Every trailer I have ever seen with one AC needed more air conditioning. Someone here is going to pop up and say their 13.5K does just fine and they don't need any more. Good for them. Their trailer is an anomaly or they just don't camp where it's hot. One trip to the Gulf Coast will help you decide. You don't know where you may possibly end up or what a potential buyer may want, and the price difference of approx $150 makes it a no-brainer, IMHO, to buy the larger unit. They both fit in the same 14" cutout.
2013 F250 XLT
I now have a Keystone Outback, but I try to help when I can.
Bitter Gun Owner