Maybe this might work ...

A random number is always a word variable.
The value is always between 1-65535.

So if the 1-65535 is considered the Decimal part, then multiplying it times the range you want (in your case 20) and take the High Word result, then add the minimum...
Code:
Value  VAR WORD
RND    VAR WORD
MinCnt CON 10
MaxCnt CON 30
Scale  CON MaxCnt - MinCnt 

RND = 1234 ; The Seed, however you get it

RANDOM  RND
Value = (RND ** Scale) +  MinCnt