Does this product have a smell? Does it off gas?

I used it outside, and did not notice any smell.

good for sealing taillight lens on my camper ?

If you're sealing between the lens and the body, yeah, it would work. This is kind of like a tar compound...it is flexible and sticky so kind of think along the lines of pushing some chewing gum in a crack. It's not going to be pretty when the bulb burns out and you have to remove the lens to change it, there's going to be some residue left. but it will seal up the crack for sure.

Is this for RV skylight?

It should work great for an RV skylight. I use it to seal windows on boats so you should be fine.

If I apply this in sub-50F degree weather, does adhesion improve when temps go up? I'm using it under a solar panel mounting bracket, with screws.

Do you mean below 50F or did you mean minus 50 F? It took me a minute to figure it out. I assume you mean below 50 but not minus 50. Anyway yes to your question if it's below 50 but if it's minus 50 I'm not sure how it would hold up to apply. As in frozen state it might be brittle and getting the film off it might also be difficult. Anyway our temps here in NW MT have surprised us and warmed up above 50 in May nicely and that includes evenings. Often if you are in a warm climate you need to put in the fridge for a day or even freezer for a few hours to help you manage the properties of the butyl caulking. Think of this as a line of caulking in tape form because that's basically what it is. Also some people use two layers and over lap ends when they come together well beyond just a smidge think more like an inch or more overlap. It can spread out a bit. Good luck with your projects and some people like to apply in cooler temps because it can be easier to handle. But way below freezing probably NOT.