[JRTJH]

Welcome to the forum curtthegreat. I think you've touched on one of the most significant factors in satisfactory (or is it unsatisfactory) cooling. The ductwork is in the attic space. It is surrounded by "superheated, unconditioned air".... What that means is that the cold air being produced by the rooftop units is actually being "warmed to near room temperature" as it leaves the rooftop unit and passes through the ductwork. Most of the ducting in the trailer's "central air delivery system" is actually 3/8" or 1/2" aluminum foil covered foam. Next time you're in Lowes, look at the R factor printed on their rigid foam sheeting. It's typically around R2 for 1/2" thickness. Add the foil backing and maybe it will insulate to about R3. Now add (at most) 1 or 2" of spun fiberglass insulation on top of that, and you'll have around R4 insulation between the cold air and the "superheated" attic air. What comes out the ducts is already "close to warm" rather than the cold temperature that's leaving the rooftop unit.

One of the "resistance factors" from buyers in the early 90's when ceiling ducting was being introduced was the inefficiency that came with it. Before that, one A/C unit, 15K BTU in size would cool a 32' travel trailer and keep it comfortable. The problem was that you couldn't close any doors or the "down-spout air" was blocked. So manufacturers introduced a ceiling delivery system. DISMAL FAILURE !!!! It was (back then) a thin aluminum ducting that ran through the rafters and by the time the air got to the bedroom, it was hot, not cool, but hot. Through "trial and error" the system evolved to what's available today, foam ducting and shorter runs from rooftop unit to vent. So to minimize the heat gain, two, three or even four rooftop units are needed. And, it's still not as effective at removing heat as the old "air dump" system.

So now, for the past few years, most manufacturers use a "combination system" that has ceiling ducting for "moderate temperatures" and an "air dump" that they call "quick cool" or some other "gimmick" brand name, but it's not "new technology". Actually, what it is, is a reversion to the older "air dump technology" from which RV air conditioning started.

Unfortunately, most RV manufacturers, RV dealers and PDI "introducers" don't even touch on the most effective way to cool the RV on hot days. That's to just eliminate the air movement through the ducts (where it gets hot before entering the cabin) and use the "air dump system". It's "not what they told us we had" (central air) but it is "cold air into the cabin" and it cools much more effectively than the ceiling ductwork.

We find, typically, that we can keep our 31' fifth wheel much cooler with the air dump and a fan to blow air up the stairs, even in 100F temperatures. When using the ceiling delivery ductwork, maybe 85F cabin temp, when using the dump, we can keep inside temps around 75 or so. Sure it's loud, no such thing as a quiet place to read a book, but much more comfortable than the elevated temps we get with the "central air".

Your experience, opening the "dump vents" to deliver cold air directly into the room by avoiding the ductwork is the same as what we've found to be true. Sometimes "the old way of doing things" is still the most efficient... It may not be the "best or the most quiet, but the most productive.....