But when done booting and WB comes up I have around 900 kb free memory and the trapdoor expansion port is empty.
There are many ways to do this, all involve hacking the hardware.
1. Agnus chip expansion. The Megachip is still available, which replaces the existing Agnus with a plugin board containing a newer (Fat) Agnus and 1.5MB. There might have been older 1MB Agnus versions of this.
2. On the Rev 5 A500s, only half of the 16 memory sockets were filled. With the correct chips, a 1MB Agnus and a soldering iron you can expand to 1MB chip easily.
3. On the Rev 6 A500s, a revised 4 (?) chip RAM solution was used. As I recall, you can still expand this by piggybacking chips on top of each other (to put it simplistically).
Most likely explanation is the Agnus expansion. Open it up, and you should see something like
http://www.amiga-hardware.com/dkbmegachip.html sitting in one of the chip sockets.
