I dont want anyone to get the wrong idea about what i said. Maybe i said it wrong. The "tent in the vent" mod will increase the airflow to the ducted vents in your rig. Some people claim it helps alot, Some havent had great success with it. There have also been several threads that dealt with resealing the main unit to keep it from drawing in hot air, trying to adjust blocked duct work, even adjusting the dead end in the longer vent runs. Different owners have had different luck with all these different modifications.
I did the tent trick when we got our new bighorn. yes, with the unit sitting there in our yard I believe it helped with percieved airflow from the vents in the rig. Was it enough? No, not even close. But we are talking about very hot temps and very high humidity for some of us. If you only camp in areas that aren't really hot and have very low humidity, You may be fine with a single A/C unit and the "tent in the vent" mod.
My opinion, and this is only my opinion, I have never been to HVAC school or worked in the industry.... On these larger Rigs, the main ac unit (15000 btu), simply is not big enough or designed to move enough air to cool the rig the way that we want it to. This is not a Heartland problem, there is nothing wrong with the way they are installing them. It is an industry wide thing. Every manufacturer of large RVs is doing the same thing. And every manufacturer designs with the thought that you will end up with a second A/C unit. Heartland has just been thoughtfull enough to include the wiring and breakers already in the larger rigs. Some maufacturers dont unless you order it.
So, if you are going to install a 2nd A/C. I dont really see the need to divert air from the rear unit through the ducts to the bedroom where I have a front A/C pumping cold air already. I would rather be able to open the air dump if I need to increase the amount of cold air being pumped into the living room of the rig.
Again, this is just my opinion, And im not saying you wont like the "tent mod". It does increase percieved airflow to some of the vents further from the main A/C unit. But if you camp in areas with high heat and humidity it isnt going to change the fact that you need more cooling and air moving ability.