No you don’t  have to replace the whole toilet but new tanks aren’t cheap so try caravan service centre and or caravan breaker for a secondhand model. 
You will need;
Manufacturer name, model,..........year and serial number ....for year and serial number look at the weight plate on tow frame and or caravan body nr door for this plate and clean plate take digi picture without the flash and you will be able to read it more comfortable rather than kneeling on the ground.  Unsure of model of the toilet? Then go to the thetford website below and look it up under the ‘built in ‘ listing, then continue to the pdf’s once you’ve established the model of the toilet and download the free pdf repair video’s for future reference. 
If you are in UK [its a world forum] and need more help on this then post your own reply to your question I’ll see if I can help further.
try www.motorcaravanning.co.uk for new
If you don't know toilet model go to thetfords site look up the 'buiilt in ' toilet list for a match. If you bought from a dealer then ask him where the cassette  is as he should have told you it was missing.
Thetford access continue after finding your model to the free pdf repair downloads down load and save to disc or computer hard drive for future reference if the C2  then add an answer to your question it's a C2 I have self compiled help sheet for this model. C3 are fitted to motorhomes or caracan with built in water tank and operate from a none return valve via onboard main pump .The C4 is a manual pump all three toilets are similar.
UK & EUROPE	www.thetford-europe.com	
Australia	www.thetford.com.au          (Even get request from Australian campers)	
	Australia site has stockist list.	
		
HOME PAGE 	SELECT 	ENGLISH {if in Australia forget this selection it doesn't come up)
	SELECT 	CUSTOMER SERVICE (top line) 
	SELECT 	PRODUCTS  FROM TOP LINE
	SELECT 	BULIT IN OR PORTABLE WHICH EVER APPLIES.
	SELECT 	USER INFORMATION     FROM LIST ON RIGHT.
	SELECT 	REPAIR INSTRUCTIONS       FROM LIST ON RIGHT
	SELECT 	BUILT IN OR PORTABLE WHICH EVER APPLIES.
	SELECT 	SELECT YOUR TOILET SERIES MODEL NUMBER FROM RIGHT LIST.
		YOU WILL NOW BE IN THE PDF LISTINGS FOR YOUR RANGE OF TOILETS JUST SELECT THOSE THAT APPLY TO YOUR TOILET AND DOWN LOAD THEM THEY'RE FREE.
	AS YOU GO THROUGH WATCH FOR 'ASK THETFORD' ON LEFT LISTINGS	
	YOU MAY NOT FIND WHAT YOU WANT AND YOU CAN SEND THEM A MESSAGE.	
		
		FOR SAFETY REASONS WHICH  I AGREE WITH THETFORD DO NOT OFFER REPAIR ADVICE FOR FRIDGES. REFER TO SERVICE CENTRE
                
 bob
                
 August 2011