I'm surprised a plumbing diagram for a Winnebago isn't available, but I'm not sure it would help you because that looks like a DIY modification to me. Having a single city water fill point and a selector valve to choose tank fill or city water isn't uncommon. The only time it can cause an issue is if the valve isn't closed completely and you overfill the water tank, perhaps causing structural damage as it expands. (The overfill usually gets overwhelmed by the fill pressure.
You could restore the system to the original, or add a separate tank fill attachment either in the wet bay or on the RV side. There are many styles and types of fill attachments available. My RV has a plastic locking door that allows city water connection or tank fill in a separate access hole to the tank. No valve to set or fail, it just works.
The missing drain could be what the 'T' fitting with the cap on the end modification replaces. Not sure about the copper tube with the flare end, can't see where it's coming from.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI