Low power device in sealed unit -- design help
A few ideas come to mind:


Your tank is polyethylene. You should be able to sense liquid level through the wall of the tank using your capacitive sensing. If that doesn't work it won't work in a PVC pipe. Advantages: no wireless, no battery, no retrieving the unit out of the tank ...
I'd consider an ultrasonic level sensor. These are used in all sorts of industrial applications, quite reliable and can go in the top of the tank where it shouldn't get dirty. If it does get dirty you may have problems. Advantages: simple, analog output for level display, easily retrieved for adjustment, etc.


I've never considered the matter, but is there any risk of methane buildup making the airspace an explosive atmosphere? If so you need to take appropriate measures with any electrical devices inside the tank. From memory ATEX equipment is limited to 10 V or so at 8 mA. (Someone will jump in with the correct numbers.) This is low enough to prevent a spark in any circumstances.