I'm a little unclear what the situation is. If the dealer says you need a neutral and you don't have one, then you need one. The unit may be 240/120. If the cable has a neutral and the dealer says you don't need it, then as long as it is hooked up properly, leaving it capped should be fine as long as there isn't excessive moisture or humidity. But the white coming off the GFI breaker has to be hooked to the neutral buss either way.
