If ur van battery is in decent nick u will be able to run a single pump for minimum 3 hrs before even thinking about ya battery, if u running 2 pumps direct u will have easy a hour stationary.
I have had the van battery run flat in the past on a few occasions in last some many years but I carry jump leads with me, it's no big deal.
Not sure we're battery is located in transit van but in trafic it's under the floor In front of passenger seat, so easy enough to run through into the back. My pumps are screwed into bottom of bulk head, so outlet nipple into hose around the back of tank through the pump then into di bottles at other end of tank