Fresh water tank sensor screws

Just get "Well Nuts". They install after you drill a 3/8' hole in the tank and when you tighten the nut, the rubber seals against the tank. Doug

http://www.amazon.com/M6-1-0-Well-Stainless-Steel-Screw/dp/B0040CYUSC/ref=sr_1_8?s=industrial&ie=UTF8&qid=1374524943&sr=1-8&keywords=well+nut

OK. LEAVE the old sensors alone and install all new Well NUT sensors next to the old ones.

The OP asked how to get them off. Having done that on my grey water tank in order to upgrade the sensors and not wanting to drill new holes, I was able to remove the old corroded ones by using a hack saw blabe mounted on a handle that allows one end to be free of any hardware. Placing a protective piece of flashing against the plastic tank housing so as to prevent the blade fron contacting the tank as I moved the handle and blade in such a way as to cut into the small rubber collar of the well nut that is positioned outside of the tank just after the screw that tightens the plug to the tank. Once cut, the inside portion of the well plug will fall or get pushed to the inside of the tank and can be removed from any of the entry or exits on your water supply fittings. Need less to say this works with the tank removed from the trailer mounts which as I understand it is already in that position as it fell off.