G'day Ian,

The Flush Tank can hold up to 15 litres, which is (as the name suggests) your toilet flushing water.  You can fill as much or as little of this tank up as you like, but if you are choosing to you a flush water additive, make sure you're adhering to any minimum ratio guidelines.
The Holding is for your waste, so filling it up with water means less room for you to actually use the toilet.  When you are adding your initial holding tank chemicals to an empty tank, there will be a small amount of water required to dilute this/activate it, but again, just adhere to the use guidelines of your chosen chemical.

If you need further information, please refer to the product manual, listed as a PDF just above the Q&A section online.
Hope that helps!

G'day Peng,

You will get up to 32 flushes from the 345 model, so depending on how much you use it you could get upwards of around 1 week.

G'day Andy,

It's not advised to use seawater for rinsing or flushing your toilet as this may cause damage to the seals, and reduce the efficiency of the chemical additives. 

You'd be ok to travel with it half-full, Peter. Make sure everything is sealed up and transport it in a good travel bag like the Porta Potti bags made by Blacksmith. These have a PVC base which adds a level of redundancy in the unlikely instance that you do experience any leaks. 

G'day Paul,

You can store the Thetford Portapotti Qube 345 in separate parts as long as you make sure that you've sealed it all up nicely otherwise you might arrive to a nasty surprise at the other end!