I have always read that the best place for the weight of tanks or large weights from the rear axle to the cabin is as close to the rear axle as possible.
Yes and no.
If you are talking about a vehicle that will be purely transporting goods over reasonably well constructed roads, then yes, that is a pretty good idea. After all why not make full usage of the larger rear axle componentry, which normally includes 4 wheels on the single rear axle.
However as this vehicle is running 4 wheels, one in each corner, then weight distribution starts to be a more critical factor. In an ideal world when really needing 4x4 driving mode, as opposed to just using 4x4 mode, weight distribution is usually a critical factor for vehicle longevity and/or the ability to extricate oneself from an interesting situation.
I was taught to drive 4x4 and 6x6 military trucks nearly 55 years ago in an extensive military driver training regime over a nearly 4 month period. Then I was sent into the field where I really started to learn and understand the dynamics involved in getting both the truck and driver in a position that enabled safe transport across various terrain without damaging the vehicle, and hopefully without getting stuck.
Although many 4x4 trucks generally have nearly double the rear axle capacity over the steer axle, the super single wheel/tyre combinations most of us adopt with these Japanese light trucks is always identical front and rear. This then dictates, to a degree, that both axles should generally be equally or nearly equally loaded. Another critical element is air pressure in the tyres when running in low range 4x4 and requiring low air pressure to traverse the terrain.
The best case scenario when using low air pressure situations is for all tyres to be running the same air pressure, this then equals out drive train forces across both axles and wheels. By doing this, vehicle stresses are shared right across the whole chassis, and equally between the front and rear axle. As a result, your engine driveline should live a longer life, and your chances of getting out of a sticky situation are usually greatly enhanced.
Mick.