Steve why dont you look at getting a 4x4 , Your solar jobs are off road so in wet muddy conditions you will manage without problems were a van would not .
A L200 for instance has a 1120 kg so you could get a good size tank in and all the gear you need , I have an L200 Trojan which has the smaller load bed which takes a 400ltr flat tank .
The L200 warrior will take a 600ltr flat tank , Load beds have plastic liners usually so put a tank straight in strap it down put a couple of pump boxes in and away you go .
You could get a good Truck with truck top for £4,000 or less . Mike