1) We have an aftermarket Pioneer entertainment center. In my limited experience (we don't listen to AM/FM much) it works OK. See #2.
2) The PO of our Sunstar removed the AM/FM antenna entirely. I replaced it in kind. If your overhead compartments are like ours, it's pretty easy to get to the under side of the middle of the roof where the antenna is and see if anything is awry with the antenna connections. It might benefit from a ground plane. The cable has a shield wire and terminal to connect to something. Even some aluminum foil tape might help. Else, you could add an aftermarket antenna on a pillar
3) The PO glued the SiriusXM mag mount antenna to the fiberglass eyebrow over the driver's seat, along with a GPS antenna, and drilled a hole in the roof for the cables, which he sealed with acrylic latex household caulk. The SiriusXM antenna didn't work very well. I left it in place, sealed the antennas and hole with Dicor, and added a marine SiriusXM antenna near the AM/FM one. It works great. I made a new cable entry into the eyebrow, and used a Winegard roof entry plate, butyl tape, stainless screws and Dicor to seal it. (The plate seals 3 cables, an amateur radio antenna, GlobalStar sat phone, and the SiriusXM.)
If you want t try the mag mount antenna, I would test it on about a 4" galvanized plate glued or taped to the roof, and run the cable in a window. A 4" electrical box cover works.
2014 Sunstar 31KE
1988 Suncruiser 31 RQ
1968 Travco 210