Found an RV shop with a SealTech machine in Cleveland, GA. Lots of bubbles around skylight and on other seams. Also a couple of clearance lights, a couple of compartments, and the entry door. Even though the entry door and compartments could not have been involved in the shower leak I had them sealed anyway. I still don't feel they found anything that would have allowed the amount of water I was seeing enter, but won't know until the next big rain.
Winnebago says the roof sealant must be checked every 6 months, and cleaned and replaced every two years. Pretty onerous requirement, especially as owners age beyond safely getting on the roof. The "semi Class A's" (these models are oriented to entry level and weekend use - compartment doors are like Class C rather than the full size doors on Class A's) like my 30T model have very flexible frames, which I'm certain adds to the likelihood that sealant will be popped loose. If I don't lock all my compartment doors they will pop open while driving.