My Coachmen class C has a somewhat similar setup. Although I haven't actually tried it myself, I believe setting the valves to winterize and opening the fill valve allows it to suck water (via the winterize inlet) and deposit it into the RV tank. The valves, etc. are even labeled "winterize/suction fill" or something similar. If you have both a winterize port setup and a single water connection for both filling and city water use, the RV maker would have to add unnecessary extra parts to prevent this from working.