To me and you, a strapped in tank logically not a modification, however the tank sits in there day in day out, unlike cargo, it can be argued that if you've screwed a bracket to the roof or floor the van has been modified, what do the insurance company accept, ??
You will only know this by providing a list of what's in the van and how it's secured then your covered - don't chance it as these barstewards will screw you over if they see the smallest of get out clauses.
You may save £200 now, but lose £1000's down the line
Darran