CrispyBones, I'm glad it worked out for you on your travel trailer. I've read other positive reviews so was interested. It's disappointing, but apparently the roofs on certain Winnebago class A motorhomes are too thin for the device. Maybe some clever DIY person has figured out a way to make it work and will post here.
Bones,
I don't think there is a way to use their kit on our rigs as we only have 3.5" of room, and like the manufacturer said our mounting bolts are in the way as well.
You brought this up in a timely manner though, it's AC season and I've been working on mine all week and your post got me thinking, so I did a little plenum modification. First I cut 2 pieces of aluminum from old license plates that fit from the edge of the chute to the edge of each duct and I taped them in place. I cut a slot for the mounting bolts then sealed it up. I taped everywhere I could to make a smooth transition. Those 2 top pieces took care of the square corners that go nowhere which I'm sure a lot of air velocity was lost when the air slammed into the dead end corners, now it can only go straight into the ducts.
Next I made up a bottom piece that consists of a Vee centered on the chute and angles down to the bottom of the ducts where I made 2 more bends to flatten the metal about 1" before where it mates up to the ducts. This piece fits tightly and the factory cover with insulation fits over it.
I forgot to take a picture of it before I installed it but what that does is when the air comes rushing out of the chute, instead of slamming into the bottom plate it hits the top of my Vee which is dead center to the chute so it directs the air to the sides and into the ducts. (you have to measure this very carefully because the chute is not centered on the plenum) I sealed everything up with Nashua 324A foil tape, if you do this don't buy the thin Nashua tape which is garbage. The 324A sticks like nobody's business and even holds up in rain, I've used it for years.
For the bottom vee I used some sheet metal I had laying around. I made my cuts with a band saw but shears or an angle grinder could work too. I made my 3 bends on the vee with the 18" Harbor Freight sheet metal brake, but if you don't have one you can just clamp the sheet between two pieces of angle iron to get straight bends.
I won't be able to test for a few days because I have the roof top unit open for an Easy Start install. I don't know if it will work as well as the KoolRV product but one thing I like is the smooth channeling of the airflow over flat metal instead of corrugated flex at an angle. I basically did the same thing as the product, I gave the air a direct path into the ducts, and only into the ducts with nowhere for it to get trapped and have to change direction and slow down. I have high hopes, heck it might even work better than KoolRV.