I own a 1981 26 foot Winnebago motor home. This unit is built on a Chevrolet P30 workhorse chassis with a 454 cu in engine. I develop avapor lock condition on long grades (4% to 6%). I also have a problem on hot days on small steady grades. I presently run a Carter 4070 electric fuel pump in conjunction with a OEM manual fuel pump with a necessary return line for a fuel delivery system. Anyone have an idea how to correct my problem?