You could still have the porty truckmounted without a water tank, long as you have got an auto feed and dump on your machine ,and where it is possible just plug into the water supply and away you go ,carrying water on a van is a very hazardous thing to do and unless your water tank is set up to the last detail to avoid errors you could end with big problems,in my first days of carrying water i had a flood in the van where a connector had popped off,ended up with crushed equipment,weigt transfer and safty are a priority,and stopping the water splashing through the lids is not as easy as it sounds ,sorry to seem a bit negative but these are the hazards with do it yourself setups.
cheers mark