If it is like the 2004 Autoquest 100 we had then the pump is accessible by taking the wide cap off the top of the tank. It's awkward to get at though and I was glad I never had to change it.
The gauges in those vans are notoriously unreliable. Even if you do get it to work properly it will only be for a short time. We simply got to know how long a tank lasted us and topped it up in time before it ran out.
In our van the pump was mounted relatively high in the tank so there was always quite a quantity of water left in when it was apparently empty. You may want to mount your replacement pump slightly lower (if possible) if that is the case with your van.