The 2nd a/c will be a flow down / directional (ie no ducting). You could always pull the ceiling apart and tie it into the ducting there but it would probably be too much output for the single bedroom duct that is already there. I love our bedroom A/C and often times sleep with it running.
Any time you can get more A/C into an area, you will cool it. Remember, Toy haulers are not that energy efficient with their slides, single pane windows, and all of their "holes". Some people have gone through them and sealed up what they can but if you are parked in the desert, good luck.
I have 3 A/Cs stock duct system, roof mt in the fbr and a free standing unit in the garage where I tied the heat exhaust into the existing dryer heat outlet since I won't use a w/d works great will freeze your butt out. I sleep back there since the wifes a nurse and works nights she has a hard time sleeping so she's up a lot but I need my beauty sleep lol I also tinted all my windows with Llumar N1020B keeps 77% of the heat out, made with copper and works great with the factory glazed tint, makes it hard to see in as well
Let me start by saying most larger TH have a primary a/c with 15,000btu capacity and the secondary master bedroom unit is 13,500btu. Most larger TH have ducted pri units and non ducted sec units. Although have seen some large 5th wheel th that have a separate duct system to do the master bed and the bathroom/hallway and a vent in the kitchen area also. Either way, together they supply 28,500 btu of cooling to your TH. More than enough for most extreme conditions short of being in the desert direct sun under no shade mid day summer.And even then it still would be bearable, although 72* probably not possible.
To answer your question, unless there is a separate duct system for the second a/c unit, connecting it to the same ductwork as the primary a/c unit, while its(both)running, will be a problem for sure.
Each a/c system needs its own ductwork in to order to run at the same time.
Anyone tells you different, they don't know hvac... for sure.
Here is why...
If you are going to try and tie into the pri units supply duct in the master bedroom, it will be like trying to paddle up the creek without a paddle, pissing against the wind, etc...
You will cause the primary unit to run at a lower less efficient superheat(by cutting down its airflow), and cause the secondary master bedroom unit to maybe freeze up, because of a large lack of airflow from connecting its total output to the 1 supply duct in the master fighting the primary unit.
A/c duct systems are sized to carry a specific amount of air (cfm) at a specified pressure drop so that the unit can supply the full amount of cfm and specified btu.
In other words, you wont get the benefit cooling you are looking for.
NO WAY you will get the combined 15,000 btu + 13,500 combined cooling of 28,500 by connecting the sec unit to the master bedroom supply duct. And, will maybe/possibly damage 1 or more of the units running connected to the same duct system.
* This post was
edited 06/26/12 11:26am by allcool *