Yes turn signals work properly on the yellow.
Does the yellow light flash with the turn signals but not the red? Also are they the same on both sides? While it does seem to be strange, I wouldn't think a previous owner changed the wiring.
I actually did that once with interior sconce lights. The new LED bulb would illuminate at random. I removed the bulb cleaned the contacts, then tried to clean the fixture contacts with a pencil eraser. That's when i realized my error.
My sconce lights use a double contact bulb, but it's only got one filament, and the pins aren't staggered like on an 1157 bulb. Apparently they do not ground through the sides of the socket like the tail light bulbs do.
Does the RV have one of the flat, 4 wire trailer connectors to tow a trailer? If it does, that type connection shares the same lead for brake lights and turn signals, a wire for left and one for right that operates both. That is because small trailers typically use one brake light flashing as the turn signal. If that is the case then the wires being connected in that way without the use of a blocking diode would give you the symptoms that you have.
Sounds like it. All they had to do was use a standard signal combiner module in the wiring.
All 4 (2 left, 2 right) bulb sockets behind the red lenses ARE dual filament sockets (3 wires to socket).
