Tire Question;

I went ahead and had the tire changed with the spare. I'll get a new one for the spare.

Glue will do nothing.

Having just blown a tire on the grapevine going 45 mph, replace it. The tire took out the plastic on the side of the 5er and scuffed it badly and will need a lot of cleaning....

Replace it.

Get a new tire, don't take a chance. If it blows out, it'll probably damage your camper or worse.

Duct tape?

The outside layer of rubber does basically nothing, so just cut the loose pc of rubber off. If more comes loose, then it has a bigger problem and needs replaced.

You didn't say "cut". You imply the first signs of decomposition of the tire. If these are trailer tires o your rig, scrap them and go to LT tires. Check the deductible on your insurance policy to help you decide to upgrade. On the I-10 I saw a big rig trailer recap blow as a brand new RV was going by. I stopped as a witness. I gave the victim my mailing address. She later reported the damage to her Navigator amounted to $11,000 plus a couple of thousand dollars for hospital bills for her small boy riding in a car seat. That impressed me. Forget about the damage potential to your own rig and start thinking about liability for a multi-vehicle incident. Is it worth it?

I gotta continue this...about an hour after I got done filling out paperwork I heard a terrible noise so I turned down the radio. The noise slowly psssed me. It was a lady in an old SUV. Sticking out the filler pipe was a nozzle with about ten feet of gas station hose still attached. I had to pull over onto the shoulder I was laughing so hard. A couple of minutes later a couple of Blue Arizona Highway Patrol cars roared past, lights flashing, sirens yelping. I turned off the freeway a few miles later toward Ajo. Man! It's a nuthouse up there!

The tires tread does take a pounding but its sidewalls that do most of the flexing as it rolls along.
I doubt glue would last long as lots of stress in those sidewalls.

The outer wall is where it gets the strength. The recommendations don't change.

First, thank you everyone for your advice. I will replace as soon as I can.

But realize, I think I miss-spoke at first. It isn't really a cut to the inner tire, it's really only a crack in the rubber that doesn't go very deep; maybe an 1/8th inch deep at the most by an inch long nick.

Nothing besides the outer layer of the side wall rubber is compromised, and it's not compromised very much. Wouldn't a glue be just about as strong as the rubber? Especially on the side? After all, the bottom of the tire that takes the worst beating is patterned with grooves, and this is really just an unplanned groove in the side rubber. It's just in the form of a flap.

Wouldn't it at least be better when I pull in a few days to an RV facility to first re-enforce it with some kind of glue?