I’ll assume you get 10 mpg towing at 62 mph.
So your truck Is burning 6.2 gallons per hour. ... 19683 grams per hour.
According to the BSFC map you posted your engine will be running at about 205 grams per kwhr and producing 96 kW of power.
This corresponds to 535 Nm and 1715 rpm ... can you plot that on your map.
If it takes 96 kW to tow your trailer at 62 mph it will take about 129 kW to tow it at 70 mph.... 636 Nm at 1936 rpm. Can you plot that. Then plot 510 Nm at 1660. If you connect the dots that line will represent your average engine output and fuel consumption running in 5th gear from 60 to 70 mph.
For 6th gear plot 1270 rpm and 667 Nm; 1312 rpm and 699Nm; and finally 1482 rpm and 831 Nm. Join the dots and this line will represent your average engine output and fuel consumption towing in 6th gear from 60 to 70 mph.
