This tape is the cat's pajamas. I would never own an RV without having a roll on hand. It's permanent, completely leakproof, and it will stick to pretty much any surface.
I first discovered Eternabond many years ago when I noticed that I was having to get up on the roof and re-caulk the front- and rear-cap seams on my Class A every year or two. I had been using Dicor self-leveling caulk, but it kept cracking. Traveling on a rough road is tough on those seams and the caulk that seals them. I put 4-inch Eternabond tape over the seals (without removing the existing caulk) and haven't touched them for years. I didn't run the Eternabond all the way to the edge, so you can't see the tape from the ground. Except for a little dirt, the tape still looks like it did when I installed it.
A few years ago, I misjudged the height of an overhang on a neighbor's manufactured home when leaving a tight RV site. A bolt hanging down from the overhang cut a three-foot gash in my roof — Eternabond to the rescue. It took about a half hour to clean up the surface and apply the tape. It's been fine ever since. While I was there, I put Eternabond tape all the way around my shower skylight — a common site of roof leaks.
The link above is for a 50-foot roll of 4-inch-wide tape, which is what I like. It comes in thinner rolls, but the 4-inch version is not that much more expensive and you can cut it down to the size you need with a pair of scissors. A smaller width wouldn't have worked on the gash in my roof, so I was really glad I had a roll of that size.
You can buy a 2-foot, 3-inch-wide, piece of Eternabond for about $10, but it's about five times as expensive as the 50-foot rolls. If you're an RVer, you're definitely going to need more than 2-feet eventually, and with a 50-foot roll, you can spare some to help out neighbors with leaky RVs.
Store the roll of Eternabond in a freezer bag. In hot weather, it will leak a little black gunk at the sides and make a mess of your storage area if it touches anything else. It comes in white, black, and tan.
It's a pretty simple process. You clean the area, cut the Eternabond to size, peel off the backing, lay it down, and press on it. That's it. I've done all my Eternabond installs solo, though it wouldn't hurt to have a helper to hold the end of the tape to keep it from folding over and sticking to itself. It's hard to get apart if it does that, and you have to be careful not to let it stick where it shouldn't on your rig because it really doesn't want to let go.
You'll want to have a solvent, like mineral spirits to clean things before you put the tape down. Never put the solvent directly on a rubber roof. Always put it in a rag and clean with the rag. Be sure the solvent is completely dry before applying the tape. You can also use it to clean your hands when the job is done. They'll likely have some of the black adhesive on them. You can by a cleaner and a primer from Eternabond, but I really don't think they're necessary as long as you clean the area well.
Scissors work the best for cutting it, but don't use a pair you care about. You can clean them up with a solvent, but it's a pain to do. It's not absolutely required, but it helps to have a little seam roller. You could use your fingers to press the tape once it's down, but the roller does a better job and makes the job go much faster. Don't get a plastic roller or one with plastic parts — it will break half-way through the job. I have one with a wooden roller and handle, but I think a rubber roller would also work fine. In a pinch, you could probably use a beer bottle.
There's a nice video here showing the process.
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— Bob Ray