I need some sort of sensor, to detect liquid levels in a tank.
I had planned on ultrasonic, but that is going to be very difficult to implement physically.
I was thinking maybe the touch sensor input could be used?
So I’m just wondering if anybody here is a lot more knowledgeable about them than I am.
Well, I had the same problem (replace oil tank stick manual measurement). Also thought about ultrasonic, but was not doable (echo in tank too chaotic). I excluded touch because oil would have been too aggressive. And: better no electricity in an oil tank 
Last plan was: put pressure sensor in closed top of pipe and sink the pipe to the ground.
Unfortunately for you I now have a heatpump 
, so no details about that, just an idea.
Wouldn’t work for me as the tanks are vented. I didn’t think about echo in the tanks, interesting.
I currently have some capacitive sensors, that glue on to the side of the tank. They work, but A) a bit expensive and B) have only specific levels.
I was thinking maybe a capacitive strip on the outside of the tank. Kinda like the glue on ones, but a longer strip that would give an analog level. I’m just not sure about the wire runs, I’m sure that would throw the proverbial monkey wrench into the works.
Maybe a project for next year, too much to do this year.
Maybe you can use a IR Distance sensor for example this one:
Dutch shop, also in English
There are sensors in different ranges.
Edit: link sensor was out of stock