I have two items, each of which may be non-sense considering your situation, but I need to mention them.
1. If the 5 heaters are controlled so that only 2 or 3 run at the same time, 50 Amps may be enough.
2. When you say 50 Amps service, do you mean there are 50 Amps available, or is this the trip rating of the secondary breaker? If it's the trip rating, the transformer may not be an 18KVA, but only a 10KVA with a full load capacity of only about 41 amps.
Anyway, those are my thoughts for what they're worth.
