750 watts is an approximately 60 amp draw from your batteries assuming your batteries are fully charged and the inverter is 100% efficient. Most inverters are actually about 90% eficient so the 750 watt burner will draw about 67 amps from the battery. A typical GRP 31 battery has an amp-hour capacity of about 115-120. Assuming a 120 amp-hr capacity and only drawing the battery down by 50%, you could run the 750 watt burner element on high for about 55 minutes. At half power, you could run for about two hours. Of course, amp-hour ratings are done for a 20 hour discharge, which means a 6 amp draw for a 120 amp-hour battery. If you draw more than that the available amp-hours from the battery drops significantly. I would expect that you could run that 750 watt burner on high for no more than 30 minutes or at half for about a bit more than an hour before your battery would need recharging.

If you really want to use an electric cooktop, then you need to buy a generator or have several hundred amp-hours of battery capacity