Here are a few things to consider. First, an RV air conditioner is only able to lower the temperature of air passing through it by about 25° F. If it's 100° inside your rig, you're not likely to get below about 75°. In other words, keep your expectations moderate.
An easy test for whether the air conditioner is working properly is to put a thermometer at the air conditioner's intake, measure the temperature there, and then do the same at its output. With the thermostat turned all the way to "COLDER," you should see a difference of 25° or so. If you do this on a hot day and only see a ten degree drop--or no drop at all--then there's probably something wrong with your air conditioner.
"One of your articles I read says to always run it on high to avoid condensation."
Condensation isn't a problem per se--that's normal, a result of the air conditioner removing excess humidity from the air. What is a problem is when condensation builds up so fast that it freezes on the cooling fins, blocking airflow and bringing cooling to a halt. You can usually tell when this has happened, because the sound becomes very muffled and not much air comes out even when the fan is on high.
Four things can lead to this condition: extreme humidity; turning the thermostat all the way to "COLDER," so that the compressor runs all the time; a low fan speed that results in low airflow; and (most common of all) a clogged filter that reduces airflow. When the weather is really hot and you're running the air conditioner almost around the clock, dust builds up remarkably fast, and you may need to clean the filter every two or three days to keep the air moving freely through it.
The remedy for a freeze-up is simple: switch the air conditioner to "FAN" and let it run as long as necessary--typically half an hour or so--for the ice to melt and the airflow to resume its normal volume. Meanwhile, you can be cleaning that filter. :-)
What if you need to replace your air conditioner? Well, the Lazy Daze factory is currently installing a 15,000 BTU Dometic air conditioner/heat pump--their spec sheet doesn't give a model number. However, any RV air conditioner will fit your rig, as they are all designed to go in a standard 14" x 14" RV vent opening. So if you see a good deal on a Coleman Mach air conditioner, you can buy it with confidence. You may or may not want to spend the extra money for the heat pump feature, which is only really usable in cool temperatures--not in freezing winter weather.