Whether you need a loop--vented or maybe not--in the line between the toilet and tank depends on two things: distance from toilet to tank and whether it's an uphill run from toilet to tank. You definitely need a VENTED loop if the toilet is plumbed to also flush directly overboard at sea.
A vented loop in the intake, installed between the pump and the bowl, is needed if the toilet bowl or any part of it is at or below the waterline. It needs to be at least 6-8" above waterline at ANY angle of heel, not just when the boat is at rest...which on most sailboats puts 2-3 FEET above the bowl.
Hard pvc is ok for long straight runs, but there aren't likely be any on a 27' boat that are long enough for it make sense when you have to use about a foot of hose to "soft couple" the connections to anything rigid (toilet, tank, thru-hull) to protect the pipe from stress and flexing that can crack it. Although they aren't cheap, there are two hoses that won't permeate---Trident 101/102 (identical except color...101 is black, 102 is white)
Trident #101 Sanitation Hose ...and Raritan Saniflex
Raritan SaniFlex hose . Trident 101/102 is a double walled rubber hose...it has only one drawback: it's as stiff as an ironing board, making it necessary to insert inline radius fittings if it has to bend much tighter than hard pipe can bend. Raritan Saniflex is considerably more expensive, but can be bent like a hair pin without kinking.
Unless you're determined to keep a "real" marine toilet and lose at least 1/4 of your storage space to the tank and related plumbing, you might want to consider replacing the whole system with a self-contained contained system...
On any boat much smaller than about 28', a self-contained system—an "MSD" portapotty-- makes a lot of sense. The "MSD" designation in the model name/number means it has fittings for a pumpout line and vent line, and is designed to be permanently installed (actually just sturdier brackets than portables, so you could still take it off the boat if you absolutely have to), which means that although it's still called a PORTApotty, you don't have to carry anything off the boat to empty it.
A 5-6 gallon model holds 50-60 flushes...you'd need at least a 30 gal tank to hold that many from a manual marine toilet. No plumbing needed except a vent line and pumpout hose--so no new holes in the boat...and -0- maintenance needed except for rinsing out the tank--which you can do with a bucket while it's being pumped out. Total cost including the pumpout hose and vent line is about $200--a fraction of what you'd spend for toilet, tank and all the related plumbing needed. And the best part is, you have all the advantages of a toilet and holding tank without giving up a single square foot of storage space.
Think about it....
If you'd like some one-on-one help deciding what to do and how to do it right, you're welcome to give me a shout via PM. You might also consider checking out my newest book (see link in my signature)...it's a comprehensive marine toilets and holding tanks 101 manual that even includes step by step installation instructions.