The written specifications on both Dometic and RVP (Coleman) clearly state that a properly functioning RV air conditioning unit should display a 'delta T' (commonly known as the temperature differential between the cooled air measured at the duct or air outlet and the return air measured at the return air grill) should be 18*F-22*F.
This measurement is NOT dependent on the outside ambient air temperature. There are no absolute temperatures to measure. The 18*F to 22*F temperature differential is strictly related to the temperature of the air going into the evaporator and the cooled air coming out of the evaporator.
As you unit runs for any length of time, the inside temperature should be dropping as the air is cooling during this process, depending of course on your insulation factors, the outside ambient temperatures, the amount of solar heating the aluminum skin will receive from exposure to the sun and the size and capacity of your A/C unit.
Lew Farber...RVIA Certified Master Tech...ABYC Certified Master Marine Electrician
SOLAR TECH ENERGY SYSTEMS, INC.
AM SOLAR ...MAGNUM…VICTRON ENERGY...LITHIUM & LIFELINE BATTERIES…MARINE REFRIGERATION