An interesting topic particularly regarding auxiliary or replacement tanks!!!
My previous bike, an NC750X, was good on fuel, but only had a 14.1 litre tank and as soon as you wound it up on the motorway the economy dropped into the high 60's mpg..... not bad, but not good with a small tank!
I found a supplier of aux tanks and bought a 10 litre, or 2.2 imperial gallon tank from them and then had a company make me a mounting for it in order to fit it where the rear seat goes..... all well except of course the main tank is under the seat and the filler is just behind the rider seat under the rear seat....
My target was 350 miles at least, of available fuel and the final solution made that possible, actually it made the range for non IBA rides over 400 miles.....
I had only just sorted it all out when I decided that an R1250RT was a better option...... and with a RDL seat, much more comfortable for doing IBA rides..... granted it is an expensive solution, but sometimes you have to scratch that long standing itch to get the bike you have always fancied.