Don't you just love it when you get a tech that has none of the qualities of a good tech?
Doesn't know and doesn't have the inspiration to find out.
Doesn't know and doesn't have perspiration to go look!
But since it is your problem, you likely have both inspiration and will to get the perspiration going!
One thing going in your favor is that your year model has excellent info to look where exactly the lines run and that can lead to spotting potential for air to get in or out.
Parts catalog here that will let you turn the RV any way you need to get the right look:
https://catalog3d.winnebagoind.com/menu/Parts.htm
But to get down to the really correct drawing and view, you will need to know the build date as there are two for your year and model! Early or late?
But before going to lots of trouble as it takes a learning curve to drive the interactive drawings, there are several easy checks that might get it done!
You likely have a valve which sets whetherwater from the hose goes into the tank or as normal and inside to the plumbing. This valve left in the "fill" position can do what you have.
There is also a small plastic disk at the hose inlet on many RV and it acts as a very simple check valve. It is a very light spring keeping the disk pressed against the rim of the fill port and you can see it when looking directly into the port. If it gets bumped some way and turned on side or broken in half, it makes and air leak, so a quick look at both of those can help.
Check the valve is to the "normal" positon,not left on fill and look in the hose connection , maybe push gently to see it flops back down to cover the hole? Remember it's thing plastic on a weak spring, so don't break it!
If the first trip after winter, check/review if you have a chance the suction tube used to get antifreeze into thesystem migh have the valve left partway open and the cap missing or leaking air?
The little plastic filter housing is easy to crack?
Yes, there can be a leak but it is true that lots of leaks that let air in will also let water out!
This is one of the builds but may not match yours exactly!