I would mount the antenna on the ladder and route the cat5 cable in as I did for my two Ham radio coax cables. This picture shows how I got the cables into the cap, you can remove the taillights and route the cable from ladder side to street side behind the cap using whatever method you choose. You then bring the cable out under the underbelly and following the other wiring out to the back outer rear corner of the desk/entertainment slide and up that location. There is a removable board under the desk which gives you access to the cables that come in that area.
You can then bring it up into the entertainment center area or under the desk or whatever.
You do not need to cut a hole in the back cap, note the cable goes through the ladder mount and that mount can be replaced if you want to put it back to original sometime down the road.
You will need to be able to install a cat5 cable end so you don't have to make a hole big enough to put the end through.
My Satellite cables come down the vent between the kitchen slide and the desk slide, they do
"not" go into the vent pipe itself but run down beside it. You can access this area from the lower cabinet by removing the shelf and another panel. The problem with bringing your cable in there is where do you mount the antenna.
Take a look at the way I did mine as we have the same model Montana.