Make sure the water heater by-pass is reversed to normal and make sure the water heater is full before you turn it on.
Turning on a water heater that is empty in the gas mode will ruin it and turn it into scrap metal very quickly. Turning it on in the electric mode will toast the heater element causing a repair that is a PITA as a minimum.
FAA A&P mechanic, ex-RV dealer parts dept
Retired B-52 Gunner/Flight Examiner, B-52 Crew Chief
2016 Jayco Eagle HT 29.5 FBDS w/ 15,000 BTU AC, 50 amp service, upgraded counter top, thermal pane windows, elec. stab jacks, outside grill, backup camera and MORryde pin box.
2011 GMC 2500HD DURAMAX/ ALLISON