Pretty sure that you're going to use the same amount of fuel no matter the RPMs given a specific throttle position. You can think about it in a couple different ways.
If you can come up with an example where RPMs going up does not increase fuel consumption, that's a good place to start for me. The easiest example is gearing down in a vehicle when coasting down a hill. Your RPMs go up, but you're literally using zero fuel until your foot touches that gas pedal again. Maybe not a perfect comparison, but "proof" to me at least that higher RPMs does not equal more fuel consumption.
Another thing that came to mind for me was thinking about a workout regime you might undertake. Obviously working out and a motor pushing a boat down the lake are different things, but when you get right down to it, they're both burning fuel (gasoline or calories) to do work (move an object, whether that's lifting weights or turning a prop/moving a boat). If the force required to move an object goes down, you can move it further for a given amount of fuel burn. If the force required to move an object goes up, you move it not as far for a given amount of fuel burn.
As a specific example, think about riding a stationary bicycle. If the resistance is really low, you can turn those pedals a bunch of times without burning many calories. If you crank the resistance up, you'll burn a hell of a lot more calories to turn the pedals the same amount of times, or will burn the same amount of calories as before by turning the pedals fewer times. The motor is the same way - trimming up reduces the force required to turn the prop (analogous to less resistance on the stationary bike), so it can turn the prop more times for a given amount of fuel burned.
Fun with analogies.