Some things you can do that may make a big difference right away. First, as Jason said, pull the cover and clean the roof-top unit. Pay particular attention to the vanes of the heat exchanger. I like to use a shop-vac set on exhaust to blow air through the vanes and remove built-up dust and dirt.
Be very careful with these vanes as they bend easily, and the more bent they are, the less efficient they are. If they are badly bent, you can buy a "comb" specially made to straighten these vanes. It's the same one used to straighten vanes on automotive radiators, so you'll probably have good luck finding one at an auto parts store.
On the inside of the trailer, drop the cover of the AC, pull the filter, and clean it. Easiest way to do that is to take it outside and hit it with a blast of water from the garden hose. Let it dry and then put it back. A blocked filter will dramatically impact the unit's cooling ability.
While you have the filter down, visually inspect the air path above it and make sure that it is not blocked. In a small trailer like the 1550, you should get a really good air flow out of the AC system. So if the air flow at the output vents seems "wimpy" to you, you may need to inspect the ducts for blockage, too.
Some of the AC's for RV's have a heating element in them. If your unit has this, double check to make sure that it is turned off.
One more thing... if you have the standard DuoTherm thermostat, it is possible that it may be defective. Our's was, and while the fan was running on the AC, the temp settings were so messed up that the compressor was only coming on once in a great while. To check that, set the unit to COOL, the fan to AUTO, and bring the temp setting all the way down. If the unit magically begins cooling better, you can then begin to adjust the thermostat and find out how far out of whack it is. Ours was so badly inconsistent so I opted to replace the whole thing. But it was off at least 30 degrees on both heat and cool.
A replacement DuoTherm at the RV store runs at least $55. The Hunter digital thermostat replacement has been discussed several times in other threads on this forum. It is very easy to do, only costs about $22, and the unit is readily available at Wal-Mart and other stores. If the thermostat turns out to be your problem, then I highly recommend this swap.