^Oh, thanks for the link. I just have seen the photo in a trucker media without detailed research, but the
FAQ chapter on the site explains it all: when the tank is full, it lies on the flatbed, and when the tank is empty, the in-built hydraulic jack lifts it to the top position and the flatbed is used for other cargoes. That’s why these products are named
ReturnHauler instead of
ComboHauler 
.
For a long time we had only
Gallon Oil and
Chemso as a source of hazardous liquid cargoes, and Texas will have four companies of that type —
Syntetico,
Petrolucent,
Vortex and
Deep Wells (don’t confuse with Dry Wells from
New Vegas 
). I wonder will they be Texas-exclusive, or will they spread on the whole map, sometimes even replacing the older bases like
Mon Coeur replaced some bases of Darchelle Uzau?