Something isnt quite right there (unless im reading it wrong).
Yes, Force = mass x acceleration (F=ma)
so, if you have more mass you get less acceleration, more fuel doesnt give more acceleration. What i think you mean is with more fuel in the tank, therefore increased mass, you need to increase revs to give the same acceleration (up to a limit). As the revs are increased the fuel consumption also increases.
Does anyone notice much difference in mpg with a full tank and a nearly empty tank, dont think i ever have.
Nah its barely anything. Think about it, 1 litre of water is 1kg and petrol weighs nearly the same, so a 40 litre tank, is only ever at most around 40kg heavier on a car that weighs about 1100kg (with you and your belongins in it). This is about 4% extra weight, so not a lot of difference imo.