I know a few rvers that we travel with and the others we see often that has 6.7 and I have never had any of them claim 20 empty, 17 maybe 18 but that's the highest on a good day. Remember there's a difference between posting something on the net and talking to someone face to face.
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2013 F350 Lariat SC DRW 2WD 6.2 4.30 Gears Air Lifts 50 Aux tank
2003 F150 SC XLT 4x4 4.6L 3.55 gears
2003 HH Premier 35 FKTG