Whoever decided it was a good idea to put holding tanks in walls should be hung by his testies!!!
, they were added to the designs as an afterthought and they took up a nice locker in doing so, Having them high up , means they can gravity fed out. I mean 99% of the ones Ive used were just rigged as through flow!!. ( which is what I suspect Benny had in mind).
On other boats I was involved , I am now convinced that two things are necessary
one is all waste needs to be macerated at the bowl. , large diameter pipes ( 1.5-2") and pay attention to internal fittings that provide protruding 'lips' to catch waste on.
Tanks below toilets, and then macerated again and pump out using pump ( under water preferably)
After about 4 boats I got the system basically perfect
IN my experience electric
toilets are the greatest boon to preventing holding tank issues. The hand pumps just push too much solid material/TP atthe problem.