It take 5 minutes to get the gas flowing to my fridge, water heater, etc.
I would be a bit concerned about the length of wait that you indicate, especially if the coach is new to you and a '68 model. It is possible that the LP level may be low, but there are other potential problems such as a worn/defective regulator as well as damaged LP pipes or worn/collapsing pressure hoses. It is also possible that the burners may need cleaning or adjustment.
With my '64 Overlander, I have develolped a practice of having all of the LP appliances (and the regulator) cleaned, checked and ajusted by a trained technician every other year to insure safe and economical operattion. One thing that I have learned is that the first time the rig is used each season (as well as any time that the coach has been permitted to run out of LP), it is easier to get the furnace and water heater to light if all four burners on the range are lit and allowed to burn until all of the flames are an even, consistent color. I then shut of all of the burners and proceed to light the water heater and turn on the furnace if necessary. I very rarely run my refrigerator on LP - - I usually use either AC electric or 12-volt - - so I don't know whether this method would make any difference when lighting the refrigerator. I will find out with my Minuet this summer as it doesn't have a 3-way refrigerator (yet!).
Kevin D. Allen
WBCCI (Lifetime Member)/VAC/Free Wheelers #6359
1964 Overlander International/1999 GMC K2500 Suburban (7400 VORTEC/4.11 Differentials)
1978 Argosy Minuet 6.0 Metre/1975 Cadillac Eldorado Convertible (8.2 Liter V8/2.70 Final Drive)