No need for a flush water tank...
V-berth is the best location on a sailboat unless that would create a too-long run (more than about 6') from the toilet...so replace the tank if need be, but replace it with a new HOLDING tank. Specify a 1" vent and reroute it to go forward out the hull...that'll insure a good exchange of air that'll keep the tank aerobic, eliminating odor out the vent. I'll be glad to help you spec out the right fittings locations in the tank.
Solving the sea water odor problem is actually easy and inexpensive: Reroute the toilet intake line to tee it into the head sink drain line just ahead of the thru-hull. You can flush with sea water...then, when closing up the boat, after you've closed the seacocks, fill the sink with clean fresh water...flush the toilet. Because the seacock is closed the toilet will pull the water out of the sink, rinsing the sea water out of the WHOLE system before it sit, stagnate and stink. This can eliminate a hole in the boat or free it up to use for washdown or anything else.
You'll find quite a bit of discussion of both of these topics in the Head Mistress forum.
To answer your original question (which may now be moot if you don't move the holding tank)...the last thing you want is a pumpout line that has ANY horizontal run unless the discharge fitting is on the top of the tank with a pickup tube inside the tank...'cuz if the discharge is at the bottom of the tank, the entire horizontal run of the pumpout hose will always stay full of waste, which will result in a permeated (stinking) hose.
