3-stage automatic 75 amp 12 volt flooded deep-cycle battery charger

so i want to build a 3-stage automatic 75 amp 12 volt flooded deep-cycle battery charger, in a way like the Iota (LINK IOTA Engineering DLS-75 12VDC Battery Charger and Power Converter - 75 Amps )

I already probably have all the big $$$ parts diodes and transformer and heat-sink, but how would this be designed? this is for my RV

Ideas?

Unless you have access to a lot of specialized parts, just go buy the one you referenced, or a Chinese knock-off.