Look at it this way concerning the holes in the roof: If you drill a hole that is narrower in diameter than the screw you're using, the hole will be completely filled with the screw. I've been putting screws into boat decks for most of my life for one reason or another. You drill the right-sized hole, fill it with appropriate caulk or sealant, drive the screw in, cover it with sealant, and you're good to go. On a trailer roof, the last step is Dicor self-leveling sealant. I did my solar panel on my trailer roof that way. (On two of the four brackets I made sure to drill into a framing member of the roof. The other two brackets there was no framing member where I needed to be.)
Make sure you know the way your roof is constructed so that you don't go deeper than you should and so that you don't use screws longer than you should. If you can't be 100% certain of this part, then don't do it.
There is no way I'd be willing to drive down the highway at 65MPH with a solar panel attached to my roof without screws.
Concerning the portable ones - If you're anything like me you will hate the inconvenience of having to set them up all the time. With a roof-mounted panel they are up there working for you all the time without you having to even think about it.
-Speak
