Posted this on YT a while ago and got flamed so I'll try here with a little more info. And probably X-post to YT too.
(this will be a long story)

Car is a 99 4runner Limited, with the factory trailer light setup

My gas gauge has worked just dandy since i bought my truck. Measured good enough for me. anyways the other day I was towing a trailer for the first time since I'd bought the truck. And accelerating down the road, all the sudden my gauge drops to below empty and the light goes on. Of course i pull over expecting a huge puddle behind the truck. But nothing.

So I turn it off, and back on, it returns to normal, then a minute later, the gauge drops again. I continue the day with the gauge working intermittently.

Since then it has only worked when the trailer lights are connected. Leading me to believe it is an electrical issue, and NOT the sending unit. Because, when its wet, it reads perfectly where it should, based on the mileage i have driven.
Of interest, the right turn signal on the trailer doesn't work when connected my car, but does when connected to my roommates.

Any ideas on where I should start looking for this gremlin?

Thanks guys..

Again today. Plug the trailer lights in, Gas guage works, spot on too, Unplug, and a minute later it goes to empty again.

Posted this on ultimate yota too, more people on here so hopfully someone has an idea.