Refueling tank under Retrax bed cover canister, interference by bed cross member

We want to extend the range between fuel stops so we can refuel at the end of the day after unhitching 5th wheel. We have Retrax bed cover that rolls into storage canister that sits at the front of the bed. Canister extends about 13 inches into the bed, and has about 8 inches of clearance under it. Retrax canister has drains at each end that pass through existing holes in front wall of bed that are about 48" apart, a couple inches above the floor of the bed.

Considering a custom tank about 47.5x22x7.5 that would fit under the Retrax canister and extend a bit into the bed to make room for the pump (considering GPI 150) and fill spout. Tank would hold about 33 gallons so would double current range between fuel stops. Tank would be bolted to pickup bed at each end and in the middle on tailgate side. During rest stops I would top off the truck tank from the in-bed tank.

Crawled under the truck between rain showers and see there is a U or box shaped bed cross member at about the same place the tank bracket for bolting the tank tail-gate side bracket to the bed would be. Difficult to make precise measurements to use in sizing the tank so wondering if anyone has specifics regarding how far back of the bed front wall this cross member is for 2016 Ram 3500 long bed. Would use this information to adjust tank size if necessary.

In the event it's of interest, I don't find an off-the-shelf refueling tank that will fit although Northern Tool does have a 20 gallon aux tank that would fit (50x21x7). But I don't want to use as aux tank which involves cutting into fuel fill line and may cause warranty, OBD code, and regulatory issues. The Northern Tool tank only has fill, drain, and vent penetrations, doesn't have bung for installing pump. And it will interfere with the drain lines from the Retrax canister, will have same issue with attaching to the pickup bed as discussed above.

Alternative is to not bolt the tank bracket at one of three but this will violate manufacturers instructions which will put me out of compliance with regulations.

Any thoughts, experience, information would be appreciated.
Thanks

2016 Ram 3500 Longhorn Cummins Aisin 3.42 gears SRW long bed crew cab 4x4 tow package air suspension

 

You can install heavy duty rivnuts.Another option would be to use a hole saw for access to use a regular washer and nut

 

Have you tried the body builder site?

https://www.ramtrucks.com/ram-commercial/body-builders-guide.html

Has box DIMs not sure if you can use this at all.

 

I have a RDS 45 gal tank/toolbox that I use on my 2017 and 2007 2500. Spliced (Gravity fed) into the fuel hose using their kit. They have numerous other sizes. Never had any codes or warranty issues.

http://www.rdsaluminum.com/