Our van has three 85 litre tanks but only 2 gauges. There are 2 filling spots. The front fills the front and centre tank and the back fills the back tank. Get under the van, between the wheels would be best and check if there are taps on the water lines out of the tanks. There should be a tap on each line. Turning the tap so it runs with the pipe turns that tank on, turning it 90 degrees against the pipe turns it off. If you have them both open, you use the water from both tanks and when you run out, you have run out. By isolating each tank, you can get a better idea how much water you are using. We use our front tank first as it doesn't have a gauge. When that is empty, you can hear the pump stressing, we change to the centre and turn the front off. When that is empty, or near it by the gauge, we turn the centre off and use the back.
If you have 2 pumps, 1 for each tank, and 2 separate water lines, you can use 1 for drinking and 1 for other stuff. If you only have the 1 pump, you'll need to run the water through both. You may be able to spend a little time and rig up a drinking water tap running through a filter so when you need drinking.cooking water, it comes from that tap other wise you get out what you have put in.
Hopefully you have a setup with isolating taps if you don't they are really easy to retrofit.
when the weather cools down a bit I will get in the shed and have a look. We had a Windsor Genesis prior but only one water inlet with two tanks. So this is new to me.
