Hey Cos,,,, so your Idea makes me think of the Camelbak (there are other brands) refreshment bladder or Reservoir . (I use mine for mountain biking here in Colorado) but it is very flexible and 1 or maybe 2 would fit in a trailer and leave room for some foam insulation.
Link to camelback reservoirs
http://shop.camelbak.com/accessories...servoirs/l/310
Regards,
Jeff