I did the same suction cup mount for a long time and it worked fine. I only put the cable through the slide seals for the las few inches of running out the slide. Removed it when the slide comes in.
Another option I was considering is drilling a hole in the side of the slide and adding a weatherproof entry for the wire. It is basically a recessed connector that won't interfere with the slide seals, but when the slides are closed, it is on the inside of the rig. I felt better about that than drilling a hole in the roof.
Something similar to this...
https://www.amazon.com/RecPro-Coaxia...ps%2C94&sr=8-1
but they make a recessed one that would go into the sidewall of the slide, so as to be flush with the wall and not interfere with the slide seals.