The best I have done here which is usually on the side of the trailer and not on the flat roof is to mount one of these PVC electrical junction boxes that has a weather tight lid on it. Drill out a large hole in bottom of the junction box. You would want this to come out on the inside into a ceiling mounted cabinet etc. You actually mounted another PVC box here on the inside and continue your PVC FLEX hose with your RG cables inside to where ever they are suppose to go to... Then use PVC flex lines to gain entrance to the junction box from the side holes. All wiring is enclosed now and sealed from the weather to give tight seal against water leaks. All of this is extra work but will solve your water leak problem.
I do this same idea routing all of Battery DC cabling all over my trailer. Running FLEX PVC is best for the inside runs and maybe solid PVC lines for the outside runs for appearance...
LOWES makes all different types of these PVC junction boxes with multiple ports. I really don't have any problems here in our sun from UV's but that may be a problem for high sun areas.
Roy Ken