What stopped me from buying into DV burning Maleah's body or evidence was that even as dumb as he is, the smell of gasoline would be all over him - his hair, his clothes. IMO there was no opportunity for him to change clothes and shower.
IIRC, the gas can didn't hold much so it may be that he originally thought about burning evidence or as you say, using it for extra fuel to avoid stopping at a gas station. A 2011 Altima has a fuel tank capacity of 20 gallons so depending on the mpg, the range is at best somewhere around 23 -32, with an average of 27 so about 460 to 640 miles on a full tank in a best case scenario and 540 more likely. The distance from Kirkwood Landing Apartments to exit 18 on I 30 is about 327 miles. Round trip 654 miles. That leaves him about 100 miles short.
LE will know how much gas was in the car when it was found and they probably have the mileage readout from the date the car was purchased so they'll have a pretty good idea of the total mileage driven. They're probably also checking gas station footage along each possible route to see if he stopped for gas somewhere.
Anyone remember the gas can capacity?