Going with a flexible tank is a really, really, really bad idea for a holding tank
... Think of how bad the odor gets with a rigid tank, and then realize that flexible holding tanks
are 10x worse and much more prone to leaking. Part of the reason flexible bladder tanks are much worse, odor-wise, than fixed tanks is that they generally don't have any sort of vent installed in them... and they tend to produce more anaerobic, bad-smelling, bacteria than a fixed tank that is properly ventilated.
Placing the rigid tank higher than the head isn't all that big a deal. It just means that you'll have to use more water to flush the hoses clear of the nasty smelly stuff. It really depends on what kind of head you have and what kind of pump
you have for it.
A Lavac, my personal recommendation for marine heads, wouldn't have much of an issue with it, since it uses a large diagphragm pump
, which you could mount slightly higher than the tank's inlet.
A more traditional piston-type pump
head will still be able to do this, provided the seals are in good shape.
One major advantage of putting the holding tank
up high is that you can use gravity to empty it when you're at sea, provided it is above the waterline.