Shower Skylight Replacement

For my next RV repair I'm replacing my skylight in the shower as it has leaked in the past. I just pulled the plastic part off the top and it was very brittle and in much need of replacement. On my RV (National Tropical) the plastic dome of the skylight sits on top of a plastic frame with a metal flange that is attached to the motorhome. The metal flange is screwed into the roof of the motorhome. The skylight dome screws into the plastic frame.

I just spent a half hour searching the Internet and I can't find anything like it. Looks like most shower skylights don't have the frame like mine does. Most are just plastic domes that you screw directly into the roof of the motorhome and then you seal directly on the plastic.

That leaves me with two problems. I had planned to just replace the entire unit, but if I can't find the entire unit then I can clean up the old frame and reuse it. It is still in good shape. But if I do that, then I can't just use any plastic dome. I have to use one that fits over this existing frame and the screws go in sideways on it to hold it to the frame.

If I'm not being clear I can get some photos tomorrow and put them on here. So the question, has anybody replaced this type of skylight and know where I can either get an entire new unit, or if not, where I can get a replacement dome? National is long since out of business so I don't know how I could find out where they got it. I don't see a brand name on it, but I haven't removed the frame yet from the motorhome. Maybe there will be a sticker in there somewhere that tells who made it.