How low do you let your tank get before stopping?

Have auxiliary tank in bed which I can put 30 gallons in plus 48 gallon in truck gives plenty of fuel for whole day travel. Always drop 5th wheel at night, then look for fuel at night. Usually find fuel quite a bit cheaper then on highway. I am 13’5” tall and I am afraid of not finding station that can accommodate my height, so that is why I have the auxiliary tank. It less stress.