Why does using barrels have to do your back in? If the barrels are too heavy with 25 ltrs of water in them, then fill them less full and use more barrels.
Also, most window cleaning company made trolleys are really heavy. I use an old Gardiners trolley, minus all the Gardiners 'bits', with a sureflow pump, a voltage regulator and a cheap maplins remote control.
The battery can be easilly removed and attached. The trolley weighs nothing to lift out the van, the battery takes seconds to attach and then I use a 12V wale pump to fill it up which takes about 90 seconds for 25 ltrs.
When I'm not using the trolley, it stays in the van and connects up so as it can be used as a van mount so I have the best of both worlds - a trolley system when needed that is easy to use and not heavy to lift in and out, and a normal van mount system.
Apart from that, another idea for the depths of winter is to use a back pack filled from barrels.