Hi,
Using a 50 amp breaker is a really bad idea and may void your house insurance.
Voltage drop is the problem. 300 feet requires thicker wire. Artum Snowbird has given an excellent explanation.
However, you may wish to consider using an autoformer to boost the voltage. As a side benefit the grill will warm up much more quickly. Do not do this for hours and hours at a time.
My Sola Basic has three layers of boost at 110 (10%), 100 (20%), and 90 (30%).
If you were doing "continuous duty" then the best solution is thicker wire in the 300 foot run.