Katoomer, as I stated I am not familar with the Ventana, but most RV's have zero clearance within the ceiling. It is full of insulation, except for the HVAC duct.
I myself would avoid using the HVAC duct to run wires, you are just asking for trouble by doing that.
As for what I did. I took out all of the surround sound system. It was medium grade equipment (Pioneer elite HTIB), but Newmar had junked the average quality Pioneer speakers (part of the HTIB), and replaced them with garbage swap meet quality cubes.
I did not replace any of the surround system. I instead I finished out the factory speaker cabinets and used them for more storage.
The main reason I did not replace the equipment is because I drastically changed the floor plan layout. Instead of the TV facing the rear of the coach, I created a TV lift in the passenger slide, and added theater chairs to the drivers slide. For a space this small I didn't feel the effort to wire the slides for surround would be worth it.
If you are keeping the factory TV location, in my opinion I would re-use the factory speaker wiring. Yes it is ~20ga, but if you replace the A/V equipment with above average HTIB you would never push enough watts to stress the wire. I would not ever install highend theater audio in a coach because it ends up being a waste. It doesn't take much to fill such a small space.
Now for stereo sound, that is very different. You need to look at the interior of your coach like you would an automobile interior.
What I did was "repurpose" two over head cabinets to create a custom speaker enclosure. I only have completed photos, I seem to have lost my in progress photos
The original cabinets doors had a center section that I removed. Using acoustically transparent speaker grill material I covered the opening. It looks factory.
Within the cabinet I used 1/2 MDF lined with dynamat to create a box within a box. I built a sealed cabinet for nice tight bass.
Because of the rectangular shape of the space I was forced to use JBL 3 way 6x9's. I know 6x9's are not perfect, but the end result was impressive.
I am powering the speakers using an Alpine ipod only head unit. I would have preferred to amp the system, but my folk only use it at very moderate levels, so amping was not necessary.