Does it make a lot of difference to having if i have the in tank heater?
Your intank heater will be a kettle element immersion type , and you will be getting a 2000w version as thats about all 10 amps will allow ( 10amps = 2300w )
It should be very usefull as a preheater when you are setting up .
The other type of heater is an inline electric heater placed inline after the water leaves the pump . This can be positioned inside the machine or as an external unit .
You would still be limited to around 2000w in your caravan site ... and to use it at the same time as time as your machine you would have to run a cable to another caravan to avail of another 10amps to run it .
An inline heater will give a rise from your starting water temperature ( when the water is flowing through it ) .. i think about 25 degrees for 2000w and about 35 for 3000w . So it is still an advantage to be able to preheat your tank water with an immersion tank heater before the water goes through the inline .
You can always pick up an external inline later , there are 2000w versions available or adjusting the temp on 3kw versions should vary the amp load also .