I get that. I just hate the cost of the Titan ones. I saw you said you had an option that was "less" costly, which would be good- their list of $2K for 30 gals is nuts IMO. I feel the same about their extended factory tanks that replace stock tanks. I wonder if this is one of those things you put on a long term CL watch and just bide your time?
I am with you on the extra fuel and how great that is. I had a metal 40 gal tank in a K10 back 100 years ago and it was great to not be able to outdrive the truck (based on fuel). Also to be able to plan for cheaper fuel fill-ups. But at $2K (or anything thereabouts) it would take forever to pay for itself (not that that's the whole reason to get it I understand, but its always nice if you can kill two birds with one stone).
Another alternative might be a flat auxiliary tank that goes on top of the decked system behind the cab. Something like this:
https://www.northerntool.com/shop/tools/product_541559_541559?cm_mmc=Google-pla&utm_source=Google_PLA&utm_medium=Fuel Transfer + Lubrication > Fuel Tanks > Auxiliary-Transfer Tank + Toolb&utm_campaign=RDS&utm_content=349004&gclid=Cj0KCQiA1NbhBRCBARIsAKOTmUsPRSkjMfHLfeCDQIF39Jy-v2nYzujHvusLRc4sU_NgMtYb-9YZBOEaAtE5EALw_wcB
It's 9" tall so wouldn't stick up too much. You could mount the fill through the cap like Kingsize did and tie it in to your stock tank for transfer. And a lot cheaper than the spare space. Just a thought. Good luck!