my 01 does the same thing. the last "half" of the tank runs out pretty fast, much faster than the first "half". my low fuel also comes on at 1/8, but if i fill it up right then, it only takes about 22 gal to fill the 26 gal tank. i've run it down to the E, and been able to put in about 24 gal.
fill yours as soon as the low fuel light comes on, and you can verify how much gas it takes. my low fuel light is very consistent, coming on exactly at the 1/8 mark. yours will probably do the same.