Lots you can do to improve the efficiency.
Pimp My Coleman Mach 3 AC
Also, make sure you are using the unit correctly. Sliding it to 65 degrees will not make it cool better; only make it worse.
I set mine at 10 degrees below outside air temperature (so it feels cool inside but has a chance to cycle and remove humidity without freezing up); 90 degrees max if outside temp is over 100.
If your AC does not cycle, it will ice up and slow air flow making the cooling effort worse. If you want it cooler; raise the temperature on the thermostat and you should see an improvement.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW