I've towed my small caravan twice now, it's about 1 ton laden, petrol mpg towing is about 25 mpg not allowing for my initial full charge. Both journeys About 150 miles and involved hills - shap summit and lakes ,and Yorkshire for the TDF. It's important to save charge for the very hilly bits ( as it says in the handbook) the engine works quite hard otherwise . I had an electric hook up on the c/c site and managed to charge up a couple of times by plugging into a standard 13 amp socket in the van with the cable put through the kitchen window, this was a 16 amp site supply , however the van circuit is fused at 10 amp and I thought it might trip, I now know that the phev actualy draws 9.7 kWh over the 5 hours due to 20% charge always being retained in the battery and this puts the load nicely under 10 amps, I was also carefully not to boil the kettle etc, and checked the plug etc for any sign of getting hot. I also tried to be discreet about the charging since allthough the site policy had nothing to say I expect a veto or a charge may be levied sooner or later. Anyway I got 60 miles or so of free sight seeing which makes the towing mpg easier to swallow.
Mine is a GX3h and the best thing since sliced bread - as they probably say in Yorkshire.