My AC doesn't run cold.  I checked in to having it recharged, but was told you can't recharge rooftop units because they are sealed and that I would need to purchase a new one.  Also told by someone else you could recharge one, but it wouldn't last.

Is this true?  If it is, where is the best place to buy a replacement unit?

Most RV AC units have sealed cooling systems are are not made to be "serviced".  There are no ports for recharging or taking pressure readings.

Many problems resulting in a low cooling effort, can be attributed to blocked air flows. The first area to check is the main filter under the shroud against the ceiling. It should be removed and cleaned. While it is out, look above it at the evaporator fins. They should also be cleaned with a vac, but be careful not to bend or damage the fins. The ducts on the shroud or running throughout the LQ, should also not be blocked in any way.

On the outside rear of the roof unit are many condenser fins. Make sure these are not bent or blocked. They probably are quite dirty, and spraying them with a cleaner will greatly help. The main shroud can be removed and the coils cleaned, but many people are intimidated with this process. Most RV dealers can handle this procedure.

If the unit seems to be running slowly, a check of the voltage at the plug should yield about 115V ac. Anything much under ~110V can start to effect the opperating efficiency.

The last thing to check, as was previously stated, it to make sure the controls are being properly selected. They can be a little confusing if you have an installed heat strip. Make sure the temp selector is all they way to "cold",  and your function switch is at the "cool" selection.

There are many sources, brands and models of RV AC units available on line. Time spent there should provide which one to choose. The most expensive I've found are at the RV dealers.

Before you replace the unit do a little cleaning. A roof top A/C unit has two coils. The one you can see from the outside is the condenser it cools the Freon. The evaporator coil is under the insulated compartment. You will be able to see it after you remove the outer cover. The evaporator is the coil that cools the air. The condenser is easy to clean with a garden hose and some 409. One side of the evap coil is open to the inside of the trailer which makes it more difficult to do without filling the house full of water. Once the evap coil is dry apply aluminum tape to the sides of the coil if it doesn’t have any to stop recirculation of cool air through the evap coil.

Good luck! jg