Have thought about doing this. I don't think that you can use the external sensors on a steel/iron tank.
There are bolted ports/plates on top of my tanks but they appear too large in diameter to adapt for some of the available products (although I suppose one could take them off and drill the requisite hole for something like a Tank Tender
.). Otherwise, I have no idea how I would be able to drill the required holes in the tops of the tanks. Even with a right angle drill attachment, there is not enough vertical space.
Do you have sight tubes on the tanks? And, if you do, are there valves on the sight tubes? My boat has the tubes but no valves, which worries me.