It's true that hotels can be relatively expensive, especially in some of the touristy spots...however have you thought of other alternatives such as hostels? You can get some really good value ones: try
http://www.hostelworld.com/ to search.
Also, you could try renting self catering accommodation, for example. Like with the camper you can make your own food which usually works out a lot cheaper than having to eat out or eating in a hotel. Try
http://www.casamundo.co.uk/croatia/ for a selection all over Croatia (I don't think you mentioned where you wanted to stay!!) or
http://www.holiday-rentals.co.uk/Croatia/r1015.htm which are more concentrated around the coast etc.
For more information on camping in Croatia in general you could also try this website:
http://www.campingcroatia.net/..