[Dnick]

Just a wild guess, but you "may" have an issue at your gas tank; maybe fuel pump seal? If you're losing pressure when sitting, your longer-than-normal cranking might be getting fuel pressure up?

Do you smell gas at all? If you had a leak at the fuel pump & are losing any fuel, that might account for the loss in fuel mileage, BUT:

I'd say this may be imaginary, given your warning light. As others have said; see if you can't resolve that, or find out if it persists. Then you can check other elements of the fuel system back there.

 

[Dnick]

I never noticed (or honored) the "remove both terminals before charging the battery" warning. That really surprises me; and I'm thinking this is an overkill precaution aimed at those who might connect a "less sophisticated" charger that may apply a too heavy charge, or ???

Anyway, it's good to know I guess.