not quite sure why you say 5th wheel or toy hauler. Our 5th wheel is a Toy Hauler with AC vented into the rear. We do have a back porch with screened railing around it.
Now your concerns about the rear cooling I suppose it depends on how many AC units you have. We have two units and it takes the rear bay a little longer to cool if the door has been left open for a period. There are remedies you can think about. A third AC unit on the back which would be dedicated to the rear bay area or a fan to draw in the AC from the main part of the house unit the bay cools off.. All in all the rear cools to me but not as good as the rest of the house since the Bedroom has its own AC and the mail living area has its own AC. The bay only has 2 AC vents opposed to the main area has 4.