I have an Eberspacher Hydronic heater in a westfalia campervan. it's running very well . Does the coolant need changing at all ?

This would depend upon the installation. Do you know if the heater is incorporated into the vehicle engine coolant circuit?

If it is, then the coolant renewal/ top up will happen at the same time as you service the engine coolant,

But if your Eberspacher heater is installed independent of your vehicle engine ( if this is the case then you would very likely have a seperate specific coolant header/expansion tank for the heater) then you should always check coolant anti freeze/ anti- corrosion levels  independent of the engine periodic service intervals.