Okay no one else has answered up with a pic, so I will. Needed an excuse to go out to the trailer and dump the Dri-Z-Air canisters anyway.
Ignore the dirt, someone didn't vacuum after the last camping trip
The threshold total length in my 1988 Scamp is 22 1/2 inches. If you look carefully at the pic you'll see the wood was laid over a piece of aluminum. I think when I finally replace the icky carpet with lino, I'll put a lower threshold plate on....something that's a lot lower and easier to sweep the dirt over and out. And I'll probably make the purchase at a big box home improvement store. No point in paying freight on something I can get just about anywhere in my home town. A hack saw should do the job on what I need too.