Why not make up an extra Aquaroll pump with hose and trigger type shower head, complete with wiring to a 12 volt outlet.
Then you could wash down the doggies well clear of the van side, possibly using the cars electrical socket, switching the flow on and off with a trigger on the head. The pump running stalled for the time involved should do it no harm; they normally feature a cooling/bleeding spill integrated in the pump itself.