Well, you are partially correct. Just because the pump can pump 3 GPM, or 1.2 GPM is only half the factor. The other half is how big in diameter the water lines are in the camper. Smaller lines will flow less water. So even though a pump may be capable of 1.2 GPM, the lines might be so small it only pumps 1/2 GPM.
Shower head almost always water saver reducers built into them to prevent the maximum flow, even possible with a 1/2 inch diameter pipe with water behind it. The actual opening in that shower head may be 1/8 inch, restricting the amount of water that can pass through, thus taking longer to run that 1.2 gallons.
Your toilet just flows water. Whatever the valves the water has to pass through are set for, will really determine how much (or how fast) the water will flow (regardless of pump size).
So actually, taking a shower, you'll probably not see any difference between a 1.2 or a 3.0, because of the shower head restricting flow. Same is true for the toilet.
Now, if all your water lines are 1/2 inch (which most are in RV's), you may notice a difference in output at the kitchen sink, as kitchen faucets usually have no restrictions (although some of the newer ones do).
Hope this helps.