If You buy the cheep Chinese sensors, yes. Go for better stuff. Stainless as @ Paul_KD7HBsuggest, or pure carbon! Disassemble an old fashioned battery and get hold of the positive rod.
Absolute garbage once again.
The "tilt" style float which is designed for these tanks, works flawlessly.
Suggest you get one.
No need for "level" details......when it's near the top it pumps out until low level.
If the tank is installed as per regs, it should have it's own "fault" detection alarm.
If not, add another of the above higher up.
I can tell you exactly how it works. I had one in a 1,000 gallon tank for my irrigation system. The internal is a steel roller that rolls back and forth in a track when the float tilts one way or the other. One one end of the track, there are electrical contacts that are shorted by the steel roller when the tilt is one way. Contacts are opened then tilted the other way.
The thing stops working when it leaks! Chinese made. Got another made in Brazil and used it until the tank split and could not be repaired.
The Brazil unit never leaked. The whole concept is a pain, trying to get to work. The bending point for the cord has to be just right for your tank and will be a trial and error test. Not good with a black water tank!
Hmmmm... I don't really want to drill holes in the side of a tank with poop.
I'm sure it will be fine, but just not something I feel like is a good idea. Now maybe I can build this into a stand off that I put in through the top. That might make sense... but at that point, just stainless bolts on a sealed PVC pipe would probably be sufficient.
So would I. I just saw a real life example today on a hike. It was to measure the water level in a river and was mounted under a bridge together with a solar powered control and telemetry unit.
The measuring device does not come in contact with the "fluid" , the level of which you are measuring which, in this case, would seem ideal.
Yeah, I'm looking at that as an option and I like it... but protecting the sensor from even the 100% humidity that is in there I don't think "saran wrap" is gonna cut it. I'm guessing I'll need something more "robust?" than the typical ultra sonics we all get in our kits. LOL
I've also thought about doing this from the outside of the tank at multiple levels. But haven't worked out the proper sensor for that kind of thing either... same issue I belive (the need to penetrate).
I don't have a link handy, but I did work with an ultrasonic sensor that was used on heavy trucks and buses to do road testing. Might have been made by Omron, not quite sure. It's won't be pocketbook-friendly though. IIRC, they were around $250 each.
On a rolling, pitching boat, you're going to get sloshing around in the tank (do sewage tanks have swash plates?) and that will likely foul the sensor. I don't think you're going to be able to avoid maintenance.
Well, there is no benefit to using crap cheap devices, that's a well known fact.
What is also a well known fact is they are used in Aus on every septic system from one side of the country to the other and, I might add, without any problems what-so-ever.
The bending point is also crap.
The unit flexes over the entire length of the cable so the bending point is no more than a couple of degrees at the point of attachment.
The only failure is in the supply lead to the motor causing moisture to enter the motor.
Main reason is because people raise or lower the pump using the power cable instead of the supplied stainless anchor wire.
I'm not even really keen on installing it the first time... The tank is already in use. 
