WVO tank for E350 ?

[Wilburtual]

wondering if anyone has any clever ideas for a wvo tank on an E350 super duty van? its got a 40 gal stock diesel tank, wondering where i should put the wvo tank... obviously there is no bed to stash it in- is there a reason NOT to have it inside the van..? i was thinking it could be a bed tank that sits on the ground right behind the front seats and you step over it to get to the cargo section of the van, and maybe could fill it by opening the side door..

i dont really want to lose the spare tire underneath, but i guess if there is a way to put the spare on the back door or something that could work..

i do lots of long trips so the bigger the better for the tank! i know a lot of people have aluminum tanks, but is plastic ok?