I'll be glad to help you...
And he doesn't have to buy my book to ask questions!
It'll be easier for others to follow if I just copy your original post here and reply to each point:
You said:
Head is a Jasco manual pump
Tank is currently a 5 gal under the aft dinette seat. Pump out is directly above.
Tank vent currently goes to the bilge (an obvious problem)
Inlet water comes from a through hull under the fore dinette seat.
Right place for the tank? It seems a fair bit away from the head.
If the tank is within 6' of the toilet, it's not too far...but whether that's a good location depends on where the head is located in the boat--forward in the cabin or aft.
Is the tank the right size? It seems small
5 gals is too small...15 is about right for your boat, whether YOU need that much or not, 'cuz the next owner will. And you'll want a rigid plastic tank, not a bladder. Your best source is Ronco Plastics (no relation to the VegoMatic Ronco)
http://www.ronco-plastics.net They make top quality tanks for a very a reasonable price and have more than 400 shapes/sizes, over 100 of which are non rectangular. And they install fittings in the sizes and locations specified by the customer when they make the tank.
That tank vent -- r
un to rear transom or put a high through hull?
Tank vent line should be as short and straight as possible...needs to be on the hull. Exactly where depends on where the tank is.
Your current vent into the bilge is not only illegall (CG regs require that all black and gray water tanks be vented to the outside of the boat), but dangerous because the anaerobic gasses--sulphur dioxide, hydrogen sulfide and methane--created of necessity in a bladder are toxic, even lethal in high enough concentration...and methane is flammable. Which is WHY CG regs require waste tanks to be vented to the outside of the boat.
Should I install a vented loop for the inlet water?
Yes...but it does NOT belong between the thru-hull and the pump...it needs to be between the pump and the bowl, which means replacing the short piece of hose that connects 'em now. See the drawing in the installation instructions here:
Jabsco manual toilet instructions
Can vent lines (loop and tank ) be tied together and be run to a single port?
No need for a vent line on a loop. The air valve that's supposed to be in the loop only allows air INTO a line to break a siphon, nothing out. What's more, the nipple where the air valve is supposed to be is so small--1/4"--that unless a vent line on it is removed and cleaned regularly, it becomes clogged with salt and sea water minerals very quickly...turning the vented loop into an UNvented loop that can no longer function as a siphon break. And since the vent line would be an "out of sight, out of mind" thing that never calls attention to itself, nobody ever maintains it.
Fresh water for the toilet?
Don't even THINK of connecting it to your fresh water system! However, you can supply it with fresh water by rerouting the intake line to tee it into the head sink drain line...details on that are in my book.
The tank looks like it is not oriented correctly. Both inlet and outlets are at equal levels. Shouldn't the outlet be lower than the inlet
Yes.
and
shouldn't the outlet go through a trap so the tank fully drains?
No.
Btw...how old is the toilet? Jabscos have a very short lifespan without problems and as long as you're gonna replumb the whole sanitation system, now would be a good time to replace it with a higher quality toilet.
We need to talk one-on-one about where to put the tank, where to locate the fittings, venting it and a whole bunch of things. So after you've had a chance to read my book and write down some questions, send me an email or a private message and I'll help you sort it all out.
