Extending stay/ Blackwater?

Using the Thetford cassette toilet (we installed a door in our build so the cassette can be removed from the outside), I plan to increase capacity by buying more cassettes. I haven't priced them and am sure to get a sticker shock but I guess they can't be more than the cost of the entire toilet which came with a 6 gallon cassette. I will probably dedicate a under truck box to hold spare cassettes :-)

They are very easy to dump in a bathroom (or better, a pit toilet like you see at many Nat. Forest camp sites.)