If the condenser is "empty" I would suspect a pinhole coil leak, but it still could just be a fitting on the charging ports, etc. An electronic leak detector would help narrow that down.
A 5 ton unit will take 6 to 8 pounds of R22. To the contractor, R22 is about $16 to $20 per pound. To the customer, that might be $25 to $50 per pound. Do you really want to put $400 worth of R22 in a 20 year old unit?? BY LAW, you can not just add refrigerant...you must determine the source of the leak. Can you say EPA and $25,000 fine in the same breath!!!!!!
Now, given the age, it is quite likely that you can not make a 13 SEER dry 22 unit work on your old, 10 seer or less, evaporator. For sure, you could not make an R410 condensing unit work. SO, you are looking at a new system as the proper course of action.