@Time2roll - I think you've hit the nail on the head. I'm trying to reuse the existing vents but that may not be the best solution. I'm going under the rig today to look to see what's below the existing vents and access to the vents.
I'll need to run at least one 4" vent to the rear floor vent so it may be easier to run a single 6" plenum and T off it for each vent. I'll know more later today. Love the idea though.
The install manual stipulates 56 Sq. Inches of return - and the current "grill" is 216. I'm thinking I can do two things - one is reduce the return to something like 60" and two is to baffle the flow of the air so the sound of the fan doesn't have a direct path into the rest of the rig.
Again, thanks all for the ideas.
josh