We had to do the same thing to our 320RS. 1 A/C would not cut it. If it was 86 outside it would be 80 inside running non stop. Had to add a second ac in the bedroom (Penguin ll with heatpump). I installed it myself and was no proplem. What a difference. I think delivered to the door was ~$900. Worst part was my storage building was only 13ft. Had to dig 10 inches out of it and regravel
Thank goodness it wasn't concrete. The best I could measure I am now at 13' 4" on the 320. I lived with it for a year and had it checked out twice and there was 18 degree difference between input and output which is in spec. Just too much camper and not great insulation in these units. I did notice that 90 % of the NEW 320's on the lot already have the second air.