Brian, I should have also pointed out something else, when you said "excessively". If you do have both the dsi/propane switch and the electric heating element switch BOTH turned on.....and do start using an excessive amount of water, then yes the propane will kick on trying to heat up the cold water going into the water heater (this is called recovery) even if you do have the electric heating element on.
The propane heats water faster than the electric heating element. Remember I said previously, both sides have their own thermostats, so when the thermostats sense the water temp is too low, then they turn on their respective heating sources....if you have the main switches turned on.
Many campers, when using excessive hot water, do physically turn on both heating sources to aid in the faster recovery time as fonzie pointed out.
You can leave the propane switch off, and use the electric heating element exclusively...... but the recovery time will take a lot longer when using excessive amounts of hot water.
EDIT: I don't have the Suburban specs handy, but here are the recovery specs for the Atwood water heaters, so you can read the different recovery rates per propane only, electric heating element only, and both turned on together.