I replaced the five ceiling mount plastic speakers and cheap passive sub with a Bose Acoustimass 6 system, then replaced the inexpensive Sony DVD/Home Theater receiver (which admittedly was kinda high-end considering what they typically install) with a Marantz low profile 5+1 receiver and a separate Sony Blu-Ray player.
I left the wiring. It was 18 gauge stranded. Considering in my unit the wiring for all the speakers runs down the wall of the slide, out the bottom with the other wires, into the frame, then back up somewhere in the trailer to spread out over the ceiling, I wasn't going to even attempt to pull new wiring. Plus, the Marantz only pushes 50 watts per channel, which I felt was plenty in a small space.
By the way, I did a similar switchover in our previous Big Country using a 5+1 receiver. Difference there was that there were only four ceiling speakers, so I had to add a center channel and a powered sub.
Here's a link with a few pictures.