I found a natural wiring plenum in my trailer and used it to feed the solar from the roof to the basement. My trailer is a Chaparral 355.
My plenum was in the wall beside the shower, where the trailer control panel was located. (Battery level, tank levels, pump switch, etc.) I found it by removing the screws holding the control panel and looking behind it.
My plenum runs from the ceiling to the basement. It has all the wires for the control panel, plus the radio and TV antenna coaxes and speaker wires which fed into the ceiling.
To use it for solar, I drilled a hole in the roof and fed my solar cables down it.
I've also fished wires in my ceiling. By removing the vents and speakers, there are a lot of access holes to pull wires with.
Good luck.
