Losing MPG=If I let my foot of the gas-to coast to stop sign, it feels like the brakes are engaging

Almost certainly not; the brake actuator doesn't generally just start applying the brakes for you.

The feeling "like the brakes are engaging" is probably something else increasing the forward rolling resistance of the car. A matter for investigation.

Of course, if it's at all possible somebody knocked the shifter into B instead of D, that's just what it'll feel like.

 

The OP could drive around for a while and then park and feel near the hub of each wheel. If a brake is dragging (or a bearing has gone horribly wrong, but this would be really rare) one wheel will be much hotter than the others. Worst case the OP may even see a disk or drum glowing from the friction.

Also look under the car and around each wheel to see if something is dragging on the ground or on a tire.

Driving with the emergency brake on would feel something like that too.

 

My experience is Gen 3 so fwiw: if my foot’s right off the gas the car will impose a slight regen drag. You can see it on the display. Touch the gas pedal just slightly dispels it, and the car rolls easier.