There are 7 caravan companies, why not channels 21-27. Channel 20 could broadcast all companies messages, very spammy but might be useful.
Final suggestion: every 3-5 minutes the caravan's radio channel announces it's next destination, maybe along with coords. When a caravan is restocking, it sends an additional message that it has arrived. Another additional message when it departs.
Maybe too complex and a simple repeat of Caravan Leader dialog might be sufficient. But 7 channels I think is best idea.