I have a new Nissan Navara Tekna Connect model, the works, ive got a snug top on the back, at the moment im using a backpack and barrels whilst my tank is being made.
Ive ordered a custom made one thats going to fit around the arches. I will upload pics when its in. Its just over 400 litres...could have been a lot more in fairness...options are endless when talking custom tanks. I picked up a wydale 350 tank in it other day and there was still loads of room. Poles could be an issue, they wont fit front to back, even a 18ft i doubt! They fit more on a diagonal, or i manage to put mine across, it sits on the edges where bottom of snug top is. Again id have to show you a picture.
On the pickup side of things tho, i absolutely love it, ive always wanted one and glad i did! Serious comfort, nice to drive and goes well! Im averaging 35mpg in it, thats stopping and starting and on a run ill get over 40mpg, im seriously impressed with fuel consumption, i wasnt expecting it to be this good...i know big vans that dont pull these numbers. 5 seats, perfect for my family, plenty of room in the back for the kids. They all love it and so do i. Ill upload pics when mines all finished

My advice, if you like them and YOU think it will work for you...go do it. I thought it and it does work