3M Blue Painters Masking Tape
This tape is really handy. I would never own an RV without having a roll on hand. It's harmless on almost all surfaces (see warning below).
It's completely waterproof, sticks well to most clean surfaces, and you can leave it on for weeks and still get it off without leaving anything behind — infinitely better than duct tape. It's good as a temporary patch for roof leaks and cracked light lenses. It can hold wires and other stuff out of the way while you're working, or hold up electrical fixtures while you're connecting wires.
I also use small pieces of it for holding screws and small parts while I'm working on something, and for temporary labeling during a job. You can write on it with a pen to remind you which wire connects where, which way is up on a panel you take out, or to make a note about where a part goes.
Important: If you have wood-grain wallpaper (not real wood) on the inside of your rig, don't put tape on it. It can lift off the pattern leaving some ugly white spots — don't ask me how I know.
I've used it as a temporary seal for roof leaks, and once, when I had a leak on the road at the bottom of the cab-over part of my old Class C, I sealed it with this tape and left it on for over a month. It didn't leak a drop for a whole month and it came off easily in one piece. I've even used it to prevent rust when my car was scratched or dented while waiting for a chance to get the damage fixed permanently.
I like to have one roll of 4" wide tape and another that's 3/4" or an inch wide.
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— Bob Ray