If you carry a large weight, then it is recommended it is installed over the axle
Actually you could put a weight directly over the axle that is within the max payload of the van and still be breaking the law, the reason for this is that there is a max payload per axle, and if you get pulled over and sent to an Doe roads weighbridge they will weigh the load on each axle and if you are over on one axle you'll be done and fined for it.
That's why you see the commercial wfp van setups putting the tank up at the bulkhead so that the weight is distributed better over the 2 axles, and also as a safety feature, if it's up against the bulkhead that's another thing besides the straps holding it down.
Tim