Hi,
Yes keep a eye on the time that you run the pump (or use a 5 minute twist type timer to control it) or you might overheat the pump if you leave the switch on to long (or loan the truck and they leave the pump on the whole time they are driving it).
Watch the gauge. 15 gallons is only 1/2 tank in most trucks. It will allow a change from looking for fuel with 26 gallons used up to looking at around 42- 25 gallons. This can mean a change from 250 miles at 9 mpg to a little over 400 miles.
It is well worth making the modifications! Just find 12 VDC someplace under the dash, if fused, not need to add another fuse, or install a 10 amp fuse, go to the switch, then to the tank. Ground the motor, and you are done.
Yes you will need to connect the diesel fuel tanks vent lines together. Then transfer of 15 gallons (about 2 cubic feet) of air will go smoothly.
Time the transfer time, and say it moves 8 gallons in 60 seconds, you will know how long to run the pump. Easy way? Fill the 15 gallon tank until full. Run the pump a fixed time, say 1 minute, and then add fuel until it stops the pump again. You will know how much transfers in a minute. Yes it should transfer a little faster at 14.2 volts while driving than while parked at the gas station at 12.8 volts.
Fred