Bobert: I have a vested interest in this discussion, I want to come and stay.
Now, I may have missed a few crucial suggestions while ploughing throuygh this thread, which I have only just seen. So, if I am repeating what others have said, sorry.
I have been dealing with a similar problem. After wracking my brains for a long time, like you, I came up with a simple realisation: gravity cannot shift water fast along a narrow nearly horizonta of 5(or in my case 8) is nothing. To shift water fast, with a short drop, you need a wide pipe and a vertical drop. Forget your pumps, rug a header tank as vertically above the toilet as you can(with a couple of feet of drop), trickle fill that coninuously(it will be slowly) from your big tank, with as large a pipe as you can rig(with the other three feet of drop).
Having said that, it's guitar playing I know about, not plumbing. The people I would talk to are the hippy solar heated gravity fed shower brigade at some local festival(if they have such near you).
Good luck.
Greg