Re: replacing holding tank and tubing
If I understand you correctly you are replacing everything from the tank to the thru hulls and seacocks. Why wouldnt you just get the correct hose size measured to fit the new seacocks.
Discharge hoses generally are 1.5 inches- 38mm ID, and vent hoses are generally 1/2 or5/8 in-16 mm ID.
I would suggest you dont scrimp on the hoses as this is where many odors eventually come from. We used the Trident 102 green stripe hose.
There are various threads and posts on Sailnet with great information on how to reconstruct you whole system. My advice here is dont go cheap or short cut things here and plan it through beofre starting. We were able to redo ours in a weekend, and the mnost difficult part was removing the old.
We have a new head going to a Ronco 36 gallon tank. at the bootom of the tank a 11/2 inch bronze ball valve to prevent sewerage from laying in the hose, which leades to a PVC three way valve. One lead goes to the deck pump out the other to a macerator and the thru hull. We use only fresh water in the toliet but left the salt water connected to use in an emergency. Also most importantly was the vent hose which is a straight a run as possible to the outside vent to encourage aeorbic bacteria in the tank.