The water tanks are right inside the sliding door, this makes them easy to fill. This also helps keep them from freezing. By locating the water tanks between the axles, helps keep centered inside the van. It is important to keep the weight evenly distributed in your van build.
I use multiple small tanks instead of one large tank. This keeps water movement down when braking and turning. You could purchase a single water tank with baffles inside. But, that would be much more expensive than linking smaller tanks together. Also, by having multiple small tanks if one has a problem, I can use the remaining tanks.
The grey water tanks are under the van, and are kept from freezing using a pad heater on each tank. The pad heater automatically turns on when the temperature drops below 45 degrees, and off when over 68 degrees.
Note: All tanks must have a vent tube. The vent tube allows air into the tank as water is being pumped in our out. The vent tube must go higher than the tank, to eliminate water leaks.