Dry and Tough Rumproast
The Maillard reaction occurs about 280 to 330 °F depending on the food. For meats this denaturing of the proteins also results in the release of water, so the meat gets hard and dry. 

By cooking at a "low" temperature, you cook at temperatures below which the Maillard reaction occurs. Well done beef has a resting temperature of 155 to 160 °F. So cooking at 300 °F isn't low temperature cooking. 

You can search for other sites but the site Barbecue Bible defines low and slow cooking and smoking to ideally be  between 225 and 250 °F.