Hi and welcome from Maryland.
I`ve never heard of S10 tanks being used before.Maybe someone has done it or you may be on to something.

There were companies making auxilary tanks for various popular vehicles.Many were installed when new by dealers and they were a popular add-on available through RV shops.In`72,the factory offered an auxilary tank option on longbed Fleetsides.They were located on the driver side with a filler up high,no door.The tube ran down the inside wall of the bedside with a boxed cover in steel.