Originally, the word "Ute" referred to a vehicle that has the tray built into the body with stylized sides. A trayback has neither of those. Whilst a lot of styleside LWBs might not have a bulkhead, they're still a lot closer to the original definition. One with a Maruti roof is even closer.
Of course the word ute has pretty much grown to mean any passenger vehicle with a substantial external load area, because of people incorrectly using the word. Because of this, you certainly aren't wrong to call a trayback a ute but calling it a trayback would get rid of any confusion. Of course, pictures of the tank would have helped too.
Fwiw I knew what you had from the start, and I imagine most others did. Certainly anyone with a Sierra trayback would know know what you're talking about and they're the target market.
MasterA, because Maruti traybacks start life as a styleside they have the SWB style tank. Only genuine Suzuki Sierra traybacks have the tank mounted between the diff and the cab. edit, beaten.
For the sake of completeness, LJ81 tanks are different to LJ80 tanks too. Though they still mount behind the diff and are very similar aside from the filler location. Both are different to Sierra tanks.
GLWS