First I would suggest being really sure there is not a switch to change from start bat to coach but assuming you have done that and found none, this is an idea which might be worth checking for ease of getting to it and then checking to be sure it IS the wire I think. Winnebago labels wires on the side with the last two or three letters being the ID which we get the info from this chart:
https://www.winnebagoind.com/diagram...ical_guide.pdf
Click this picture to get a better view but I "think" you may find there is a Battery mode switch with a wire on pin 1 of the switch being coach power. Wire should be a 16 gauge yellow wire and labeled "LS". This wire seems to come off a 6 amp breaker in the battery compartment, so that might make it just right to tie the radio in also??
You should do some checking before accepting that as the right thing, though.
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Richard
Why no RV year, make and floorplan on MY signature as we suggest for others?
I currently DO NOT have one!