What goes down the toilet does not have any effect on what goes into the grey tank.. The toilet has it's own tank (black) and all the sinks and the shower has it's own tank (grey).
What you have to do is reduce your water consumption that goes down the drain, even when hooked to city water, but no sewer hookup.
People "camp" the way they want to, so there is no "correct" way to do it.. Just do what works for you and enjoy.
If using a lot of water is your way, great... You just have to deal with the capacity of your holding tanks..
I have a 30 gallon fresh water tank. I have a 30 gallon black tank. I have a 30 gallon grey tank.
I only use water from my fresh water tank.. That's just the way I camp and it works for me... But, I never fill up a holding tank! Again, that's just the way I camp... If that's not the way you camp, I don't see anything wrong with that either.. :)
Good luck!
Mitch