Is it possible to add an auxiliary fuel tank behind cab？

A few things

Can you haul an extra 800 lb.....?
A extra 115 gl will not be enough to get you back to ND.
Can't be plumbed in (T fittings) when you fill it gravity will make it overfill and run out of the other tanks

 

Might wanna check hazmat guide for limit you can carry before having to placard.

 

No hasmat reg on size of or number of fuel tanks you can have on a truck.
I know many drivers that have 3 150gl tanks

 

3…..and 4 tanks I’ve seen as well……was sure I seen an article once upon a time that there is a limit of how many gallons you can carry. I could be wrong, god knows I’ve been wrong before, hence my first marriage!

 

Don't know how you would go about bolting that across the frame that way. Going to need either a transfer pump or a valve to open when your main tanks are empty or nearly empty for gravity feed to not overflow.
If you have the frame space I'd put it right behind one of the factory tanks. If you wanted to get real fancy you could have the third tank cut in half and and a half added to the back of each of the factory tanks, not sure what you'd have to do legally to have that certified. Can't be much different than having a tank partitioned for an oil tank for a wet kit.

 

Have you considered just going to larger tanks?
Lets say you have two 150 gallon tanks now, go up to 220 gallons each.

 

I just went through this speccing the new truck. The dad-blamed DEF tank takes up what should be room for the fuel tank on the driver's side, and the EPU batteries take up the equivalent space on the rail on the passenger side. Unless you stretch the truck (and create a huge trailer gap), there's only room for a 130 and a 110 on mine. Which is ok when you get 8 or so MPG, but won't get your from NODAK to the coast and back.

I guess life is about compromises.......

 

Might not have room depending on how the truck is spec’d. I can only have an 80 on one side due to having a blower on that side.

 

That sucks.
I just ordered 4 new truck, they have a single fuel tank, but its large.