It could be a sign of a bad connection in the wiring somewhere.
A good first step in troubleshooting would be to move the voltmeter around to every outlet, including the one on the pedestal. You should see some that aren't affected as they are on the other leg of the 50A service. Might give you a clue to where the problem is.
If you see any of the outlets increase in voltage when the heater is on, immediately shut the heater down, shut off power at the pedestal and disconnect your power cable. That probably means a missing or inadequate neutral connection which can cause serious damage.
