Peter,
The two market leaders are TomTom and Garmin, and aside from what you get from the manufacturers, there are loads of free web-sites and web-help forums for these makes.
These additional things include the location of CC and C&CC camp-grounds - the caravan club sells a list of it's own sites - but a group of TomTom forum members compiled such a list as well.
What should you buy?
Go to a shop that has the models you fancy on WORKING display and see if you prefer the TomTom or Garmin way of working. Both are simple to use, but some prefer one over the other.
What do you need to buy?
If you only take your caravan around the UK, then buy a UK only model. Such as a TomTom ONE
If you enjoy tin tenting in France or Holland or where ever, then buy a model with STREET LEVEL mapping of Europe - something like a TomTom 710 or the new TomTom ONE EUROPE
Avoid those systems that offer "Major Roads of Europe" - like the TomTom 510 - this is of limited use - being able to find Paris is fine - but you probably want to find an address in Paris - and this is what they don't do (only the major roads).
Avoid those systems that offer "street level mapping of Europe" but only include a small memory card - the full maps of Europe require about 1 or 2 gigabytes of storage - and you would need to copy the regions you want to visit down to the memory card - and you may just forget.
I live in the UK, work in Rotterdam, and have been a user of TomTom since the very first incarnations - currently have a TomTom 700 (the previous model of the TomTom 910) bought at a heavy discount once the "10" versions came out - some are still available and it's a great system at a great price.
My wife has a Garmin, she prefers their style of human-machine interface.
And as others have said on this and other forums - Navman is not in the same league.
And you still need a paper map - maybe to get an overall idea of where you would like to visit (trying to do this on a small screen is no fun) and as a back up in case you break or lose your SatNav
Robert