I think often times it comes down to availability of space for the tanks. They may have maxed out the size for the space. OR- this is the size of tank that they stock.
You can try emailing Palomino via the web site and work with the sales manager to see what is possible (I wouldn't trust a dealer to handle the communication).
My guess - I do not think it's likely that you'll get the factory to change sizes. You likely will either have to do it yourself or have your dealer do it.
(Alert the mods- aliens are trying to abduct me!)
2018 Ram 3500 DRW/crew cab/long bed/4x4/4.10 gears pulling a 2017 Spartan 1245 by Prime Time
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