It definitely sounds like the fuel level sender is going bad, but before I spent any money on repairs, I would at least get the Engine Control Module scanned for trouble codes and see what fault was recorded. If you do have a Fuel Level Sender code stored, then you probably need one. If you have additional codes stored, they may all be related and point to a bigger problem. Some parts stores offer to scan trouble codes for customers, so you may want to call around to some of the local stores and see if they provide that service.
Also, what you may want to check, is the passthrough connector under the vehicle. Its on the drivers side. If you lay on your back and slide under the van, kinda between the driver seat and second row, you will see where some wires plug into the floor (pass through connector). Unplug the wires and see if water comes out. If it does, then you have a water leak into the inside of the van, and its draining out through the passthrough. Of these wires, two of them go to the fuel level sender and if water gets in the connector, its common for the gauge to go erratic, as well as set fuel level sensor codes, fuel tank pressure sensor codes and Evap Vent solenoid codes.
I hope this has helped and please hit "reply" if you have additional questions!