I am taking delivery of a vendor truck with a 120V, 30 amp plug on the side for recharging 3 large batteries overnight. Will there be a problem if I use an RV adapter cord plugged into a 15 or 20 amp circuit?

In my opinion to be safe you would need to use an ammeter to see how many amps it's pulling at full charge rate. They may have used the 30 amp plug for a reason!

Chances are that if you have a 30 amp plug you should use a 30 amp cord. If the charging unit is on board the truck that, would be an extremely large unit. If the charging unit is not on board, the plug would be for connection to a 30 amp 12V battery charger.
Probably should read owners manual.
Are you sure that it is for charging the batteries. Why would you need to charge batteries on a vendor truck so often that you need that kind of connection. Are you sure that it is not a connection to an on board inverter and is for supplying external ac power?