Instead of trying to improve the efficiency of the AC unit, it is much more efficient to to prevent heat infiltration. Things like being sure any air leaks are sealed. Windows are shaded or covered during the heat of the day. Park in the shade. If possible, don’t have the large glass portions of the rig facing west. Limit opening of doors, don’t put extra heat or humidity inside by showering, cooking or doing laundry in the heat of the day.
As for the AC be sure the fins on the unit are clean, all the filters are clean , and if possible, don’t create a cooling deficit by turning the AC way up or off during the day and then try to catch up when you return.