My two cents:
I have had a vehicle written off. It had tanks in the back, a split charge system, fixed hose reels, and other non standard "modifications", none of which were ever mentioned. Not when I was getting a quote nor when the loss adjuster was inspecting the wreckage. They paid out without issue. The word modification was never uttered.
Personally I believe that tanks are tools of our trade. Fixing them in is only sensible.
I would never lie to an insurance company, but vans are designed to carry stuff. Carrying a tank is not a modification, whether you've fixed it in or not.