Rick -
We have the same situation. Our rig had excessive rust on the underside from day 1. We bought it from a RV dealership in Florida. They were negligent and had not washed off the underside after delivery from factory in Iowa during winter time so rig still had the ice/snow chemical residue. That combined with the Florida heat and humid air it rusted. DW had fallen in love with floor plan so I begrudgingly went ahead and accepted the condition. Winn did do a rust treatment underspray for me and replaced some sheet metal body parts that had rusted excessively.
The brake line that began leaking was in same spot as yours. It was one of the pair of 1/4" hard lines next the the muffler. I agree, it was rusty to begin with, but sure that the heat from the muffler aggravated it moreso.
We were stranded on I-75 for 8 hrs waiting for mobile RV repair. They came, replaced it and said my rig had a special system the doesnt need to be bled. We did a test drive and brakes seemed to work fine so I was not suspicious. I now know this is not the case, they should have been bled. I plan to take rig to shop to get the other original 1/4" rusty brake line replaced for peace of mind. I will get them to bleed the whole system at same time.
Always something, safe travels....
TC&C