VW Passat 130PD - very good cars, plenty of room, comfortable, plenty of of power even as std. Remapped will give a safe 165bhp. My estate remapped has given up to 55mpg. A 4motion would be better if you're looking at caravan sites as you mention, on the std map mine's given me 54mpg.
Audi A4 130PD - same engine and running gear as the Passat, but less room inside, between this and a Passat then go for a Passat.
Audi A6 130PD - Still less load space than a Passat estate
Audi A6 Allroad 2.5V6 Tdi - Would be a very good tow car even with std map, apparently the 165hp motor is the best of the bunch, but like Bertle says std economy will be down on the 1.9PD
Skoda Superb 130PD - don't seem to be many 130pd's around looking on Autotrader, and they're around 10k, earlier models are the 2.0l models, but those engines up to 2008/9 are known to have problems.