If you had the pump battery wired to a split charge relay, running the van when the cut- out occurred would indicate if the battery is faulty or not (our experience).
You shouldn't have the varistream wired through the pressure switch - the pressure switch should be by-passed. If you have wired it through the pressure switch, then a faulty contact in the pressure switch could be playing games with your controller.
If you are removing the varistream, then you reconnect the wires to and from the pressure switch. The connector/terminal furthest away from the motor is the power supply and the inner connector/terminal is to the motor. This is how the original pump is set up. These are the red cables. The red cable is connected to the + terminal on your battery.
The on/off switch Tosh is talking about would be fitted on the red cable between the battery and the pressure switch. (So the cable run is - the red cable from the + battery terminal to on/off switch, from on off switch to pressure switch, from pressure switch to motor. The black cable from the motor goes to the negative terminal of the battery).
If you are using the latest varistream (as in the make - this is the one the indicates battery charge condition) then you should fit this on/off switch before the Varistream and switch it off at night. Our experience is that the latest Varistreams consume power even when switched off. We found that a fully charged 85 amp leisure battery lost 25% of is charge in a week of being idle. Once the isolator switch was fitted, this ceased to happen.
If you do a search on "isolator switch" under my name you will find several posts regarding this as well as a link to the on/off switch we use from Screwfix. The same switch can also we purchased from any electrical supplier and places like B&Q.
From our experience I would say this sounds as though your battery is in need of replacement. Its always difficult to identify faults like this as it could also be a poor connection from the battery to your Varistream. Places to check are at both battery terminals and your fuse holders.