I have a 5500 in my Designer. It has a 10 gallon tank and will run 20 hrs +/- depending on how hot and how much the two a/c's compressors are on. But 20 hours on 10 gallons is pretty consistent.
2006 GMC 2500HD CCSB 4x4 Duramax/Allison, Titan 52 gallon fuel tank, Prodigy Controller, B&W Companion Hitch
2010 Jayco Designer 35RLTS, Cummins/Onan RV QG 5500 EVAP, Progressive Industries EMS-PT50C, TST Systems 507 TPMS, RV Flex Armor Roof