i know nothing about these tanks--but I assume they are metal.
I like plastic for waste tanks. Plastic is not affected by the acids that can and do form in such tanks, and a leak is not a catastrophe just a bloody nuisance. Leaks
in plastic heavy duty y tanks properly secured with tie downs and good foundations are unlikely to fracture or get punctured. They have no electrical
conductivity either--and do not corrode by differential aeration.
It is possible to make one's own tankage out of plywood
as a strong permanent mold
and to line it with bonded-in fibreglass made using epoxy
I seriously doubt that the stern is the best place for tankage. It is one of the places where there is more than usual movement--somewhere closer to midships, such as abaft the chain locker is a better location. I usually like the heads to be close to the waste tankage and any pump
out, siphon-breakers and air-lock carbon filters are more easily accessed..