Take a screw out of something else that is attached to the outside (outside power outlet/clearance light/tail light) and use one similar in length and size and you should be OK. There is no weight to a cable port so all you need is something that will keep it firmly attached to the sidewall. Since you have fiberglass walls you should drill a small pilot hole first to reduce splintering where you put the screws in.
2014 Palomino Puma 25RS
2011 F-150 Super Crew XLT
Days camped in 2014 - 23
Camped 2015 - 47
Camped 2016 - 51 done - 6 more planned so far