Triumph Explorer

Well, you don't know much about these types of bikes if that's what stops you from buying one. 5.2/5.3 is pretty much the industry standard. The Ducati Multistrada is 5.3 and THE standard for this category of bike, the BMW R1200GS, is also 5.3.

The trend leads me to compare with BMW ADVENTURE (8.72 gallons) & GUZZI-2012 STELVIO NTX (32 litre)

another newer entry...Yamaha Super Tenere (6.1gall)

I had a 650 V Strom (5.8gal/22litre)...it had a range of a reliable 250 miles.
I consider that the minimally acceptable for an adventure tourer.