Having recently purchased a motorhome, I started using it with an 'I've been boating so I get the 12v / limited water etc style of living' approach. It's different, mostly because of the limited capacities, exacerbated by the low but real risk of being taken to a weighbridge - a full fresh water and waste tank can take you way over the max vehicle weight, changes the vehicle handling enough to be a nuisance, as well as not being usable without dumping grey water somewhere you shouldn't, so water management is a daily consideration. Motorhomes have one big advantage - gravity empties the tank.
Thinking of any boat I've ever hired / owned / had a ride on, there's going to be some expensive re-working required to get a grey water capability that can cope with the fresh water capacity. The obvious place might be where the fresh water is, replacing a single tank with two - but even then there would be a lot of extra waste pipe work, and maybe pumps, to get through bilges / bulkeads etc., and an obvious drop in capacity. If the tank is a structural part of the boat that wonlt help either. Another option for those with pumpouts would be to go to cassette toilet and convert the pump out tank, or just route grey water to it and pay to have it emptied more often - but again more waste pipes to route, and noting the comments re septic tanks.