I’ve noticed that as the weather’s gotten cooler and the humidity has gone down that I seem to have my fingers slip off of the strings on my fretting hand.
Is this normal? Is this due to dry skin? Should I use something on the strings to help my fingers get better purchase?
For me, it’s the opposite. As the humidity, or stage sweat goes up, the tendency for my fingers to slip increases…!!
The heavy nylon jackets don’t help 
Covered in plastic, golden sequins 
No, that doesn’t sound like a common problem. People occasionally use something like GHS Fast Fret to be able to slide around even better/more. I guess you could try hand lotion?
Besides that, check your fretting hand to see if you have a solid grip (thumb supporting your fretting finger and fingers at the right angle). On bass, most people lay their fingers relatively flat. Perhaps you can experiment with using a different part of the finger pad?
Lanolin based hand lotion and more attention to technique seemed to do the trick. I was get a bit too far forward on my fingers which when combined with my drier, smoother calluses was causing my fingers to slip. I would initially get, but then the string would mute as it slipped down the side of the string, was particularly bad on the A string, easier on the thinner strings and wasn’t happening on the E string because I couldn’t fret with that part of my finger do to the angle my finger made with the string (finger was too flat across the fret board, not curling enough to get just the tip of the finger)
Good to hear! Reminds me I should moisturise more often. 
It also really helps prevent cracks in your skin, which I frequently get at the front corners of my nails during winter months. If they occur on my plucking fingers it is really painful to pluck and can throw off my rhythm as I’ll not use that finger or play weakly with that finger.
Yet to see if it causes and faster corrosion of my strings, but new strings are a small price to pay versus not playing or playing badly.
Maybe you can try a ‘fret sock’ like Scott’s Bass Lessons uses.