Well I went just before noon time to Home Depot to buy a 4 1/4" hole saw, got back and after lunch I drilled the dinette side board out taking care to insert a backer between the fabric cover and the 3/8" plywood.
The hole was done and I pulled the spare 4" duct through and installed the existing register from under the aft bench.
I ordered a new register with a shut off damper to re-install under the bench seat and dinette table.
I will keep that register closed most of the time as its really not needed where its placed under the table.
This was the easest install so far only because the duct was already installed by the factory!
And as a bonus I found a handfull of extra screws of various sizes under the dinette seat...Thanks Trail-Lite!
Oh one more surpise under that same aft bench, thats were the optional sub woofer is installed.:E