6mpg. 30 gal tank

You may be able to swap out your existing tank for the larger one,the one on my Class C on a Express cut away came with a 57 Gallon tank.

Maybe look at a RV junk yard,they may have one.

Years ago it was very common to get fabricated fuel tanks added. They would size them to fit anywhere. There is so much liability now it is probably more difficult to get one fabricated. They make diesel tanks for PU beds but they indicate "not for gasoline use" The solution might be to draw up plans for a tank yourself and not indicate it's specific use for gasoline.

Neither Transfer Flow or Titan have larger fuel tank options for your vehicle. and of course, you could not install an additional fuel tank in the passenger compartment of your van like you could the bed of a pickup truck. Looks like it's time for a new tow vehicle ;-)

Many have installed a second tank at the front of the bed and have a valve under the seat to switch between them.

Transfer Flow has replacement fuel tanks for trucks. Not sure if they make one for a van.