Careful!! When you speak of 18 gauge wire and 20 amp fuse, my first thought is it was added by a previous owner without too much thought!
Of course that could be totally wrong but when I think of dash 12Volt outlets, I tend to think of them as fused pretty light and not 20 amp!
My thought is that it may go just about anywhere that a previous owner might have found??? Or does it look like original?
On the switch, I suspect that is the switch for changing radio power from coach to start battery and it does likely go to a relay. I tried to tie into that as a handy spot for both battery supplies as a quick,easy point to add a small volt meter on the dash to watch as I started the engine. But the dash switch also made the polarity change and I blew up the two dollar E-bay meter before I figured it out!
On a 2015 Vista, the relay was way back inside on the firewall where I could hear it but never saw it before trading the RV!
Sometimes there are mysteries that I never solve? The RV after 2010 do not have the complete wiriing schematics that pre 2010 do. Without wiring charts, those mysteries are much harder!