Engine Gas mileage does not drop while driving
The driving engine use the amount it always has, the extra fuel is generator usage, don't confuse the issue
Unless some appliances are hard wired to extra External circuits
You can only use ONE power source at a time
Either shore power or the generator, Not both
The exception would be to put an extension cord out the window to a portable generator, while using shore power
Main RV power always goes thru a transfer switch usually automatic, but sometimes a manual plug in arrangement, only one source can be used, except as noted above
