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 HH Premier 35 FKTG
Retired 2000 F350 Lariat SC DRW V10 4.30 Gears BrakeSmart
Air Lifts 4X2