Outlaw,
The waste tanks do not have an overflow. Either the tank is cracked or a connection (in, out or vent) is leaking. When I replaced the black tank in my Jayco I cut the top pipes and unbolted the drain valve. The new tank I used rubber grommets made for the purpose with black silicone automotive gasket sealer and short stubs. Fernco couplings completed the connections.
For the sensors you can get Horst Miricle sensors. Leave the old ones in place and put the new ones nearby.
The waste tanks are mostly black ABS. Cracks can be repair with black ABS cement (from a big box store) and some "boat cloth" fiberglass which can be sourced from a rust repair kit or marine parts place. I fixed my old black tank bottom and it held up great until the top cracked-thus the replacement.
For the fresh tank you could install a 6" dia deck hatch to gain access to the inside of the tank for a good clean out and sanitizing. I put one in the top of my old Dutchmen FW tank but the whole tank was accessible under the couch. The deck hatch is water tight for incidental contact but I would not put one on a side. https://www.amazon.com/Seaflo-Round-Inspection-Hatch-Detachable/dp/B01ASBBZXQ/ref=sr_1_16?s=sporting-goods&ie=UTF8&qid=1501243588&sr=1-16&keywords=deck+hatch
Great work keeping this one going!
