For the SeeLevel sensors, is 5 inches the shortest the sensor strip can be trimmed? I thought I saw where they can only be trimmed for 6" length and that would mean a 7 inch tall tank, atleast. SeeLevels sound good for something that is in a protected area like the freshwater tank but something that is out in the weather like the black and gray tanks I would always worry about the sensor falling off. Maybe they wouldn't but I'd still think about it.
There might be other external style sensors that are not limited by tank depth. I think I saw one that had four seperate stickon sensors and they could be staggered to fit any depth tank, I think.
For me, in the end the best solution is to keep an eye on tank levels as well as I can without relying on sensors. If I have to worry anyway then I'll worry productively, LOL!
If you do install a new system do post about it, with pictures.
Edited to add: If the SeeLevel can be trimmed to 5" there might be enough room if they are mounted by the drain opening. I never did see how much of an angle they could be installed so that might allow for even shallower tanks.