I think it is all to do with where you are going and if water is available and if water is plentiful only have the front tank full as leaving water in the rear tank can only increase the risk of stability issues with the front tank empty.
Also having some water makes it easy to stop for tea as water is readily available.
Travelling with a water tank say half full or maybe half empty splashing around can sometimes create stability issues with a van that may not be that well balanced.
I always travel with tanks full if possible mainly because we never know where we will be that night. Also the heavier the van the less overall effect you get when towing as the weight of the water is not as big a percentage of the total weight of the van.
JR
