Circuit apparatus for interconnection between an electric power source of expected variability and an recreation vehicle requiring a known fixed range of voltage change. A voltage sensor (18) for producing signals representative of corresponding source voltage variation ranges. A transformer (60) with tapped primary (56,58) is selectively switched to provide low boost or high boost to compensate two low ranges of the source voltage connected by a relay (54) energized by the voltage sensor signals (18). If the source voltage exceeds the low boost range, a further relay (28) energized by a sensor signal connects the source voltage to output terminals (40,42) for ultimate use.