The problem with van insurance is . . . . the tank in the back of the van ! Normally you would go on a price comparison website, and find the cheapest quote. . . . . .but when you contact the company which is the cheapest they will normally not offer cover because of the size/bolted down or strapped tank.
I had to change insurance companies twice because of changing tank in the van ( which cost me £100's in cancellation charges ).
Best advice I could give is ls to give yourself half a day to research all the companies offering the correct cover,
and cheapest price.
And remember if you fail to disclose EVERY single detail of the van and tank your insurance will be invalid !