My Duo-therm Brisk-Air by Dometic rv ac wont cool.(The fan runs )

Usually just listening closely will give you a rough idea where to look for problem. When you have the fan on and switch to cool, (with temperature turned down low obviously) do you hear any type of "moaning" sounds from AC unit, or see lights in RV dim slightly indicating that compressor is trying to engage? Is the breaker tripping when compressor tries to engage? Do you hear a "click" from AC unit when switched to cool? If none of that happens, then I'd start thinking thermostat, or compressor relay. Normally if it's a faulty capacitor, the compressor will try to engage but will make a "moaning sound" and usually trip the breaker due to excess current draw. Someone experience in electrical should be able to test relays etc, but whoever does work on it, should be aware that those capacitors, as well as hydro in general are very dangerous to work with. The ("zap",- Shock) that you can experience from a capacitor or hydro, can be fatal.