I am working on this myself for my '18 Imagine XLS 22RBE. My issue is cable length from the roof down to the front compartment where my solar controllers is going to be located. I am lucky with the 22RBE because you can simply phish the cable down the refrigerator vent about 3/4 down the sidewall of the coach. Looks like I can drill a small hole from the compartment where the fridge electrical is on the outside to allow me to phish the wires the rest of the wall to under the trailer. From that point I would run them up to the front compartment.
With 12v systems the longer the run the more the voltage drop. You can loose much of your solar production with long runs. For me that would mean basically run the wires front almost the front of the camper to the rear, down the wall and then back to the front.
What I am going to do and what you can do with the lack of the vent is to come down the void on the interior wall. Don’t be afraid, it is very easy to install. For me I just located the vent pipe which is going to be in-between studs. You can press on the wall and feel where the studs are, just measure the next void between the studs from the inside of the trailer. Now what I have done is measured the distance from the middle of the vent. Go up on the roof, the vent pipe gives you the middle of the wall front to back and the measurement the distance from the center of the pipe to where you want to drill.
What I am going to do is use butyle tape making a loop around the underside of the above solar entry:
Then I will top over the vent with some self-leveling dicor:
For added measure I will cover over the dicor with eternabond tape. This is a added safety factor to make sure nothing can cause a issue with the waterproofing you have done with the butyl tape and dicor. It is also great to use in small pieces to hold down your wires to the roof.
This is the process I have used for other vents and roof penetrations on previous campers that I have owned.