Hi Dan,
Haven't used 8052 BASIC for many years. However, I've looked out some old notes.

Have you checked the value of 'MEMTOP'? This is set by BASIC to hold the last available address in external RAM.

Look at 'VARTOP' too - this tracks string variable storage. Normal variables start at 'VARTOP-1' and grow down thru RAM.

Hope this helps.
Derek.