Water Tank Level Sensor is Leaking

If it's the polyethylene that is leaking, that is a much bigger issue. Maybe you could use a Uniseal slip fitting to block the existing fitting/opening by cutting a larger hole with a hole saw and inserting the fitting. Then drill a new hole for a sensor next to it. I think I've seen a similar mechanical type repair fitting somewhere but can't find it. The Tank Depot has them for $7. Harbor Freight has a plastic welding kit for $65 which some have used to repair RV tanks but may take some practice to apply the right amount of heat otherwise could still leak. Best thing would be a spin weld repair. You can buy the driver tool but is $200 plus you'll need a router to drive it. Icondirect has spin weld fittings and drivers here.

Probably best to go to a tank repair shop. But if it's under warranty, get it fixed by the dealer if it's the tank poly that is damaged.