Mate that sounds fantastic, except for the modelling bit, maybe pics of fish caught and Tojo's recovered will be more to my liking!!!!
Pity you live on the other side of the Big Island, I would gladly give you a hand to sort out the electrics, takes time to install it so it gives you no troubles. You will get there.
There are some good sites in the US for water tanks, Dad got one for his campervan delivered for a great price and good quality too. Not sure about behind the seat jobbies. Will put site in an edit here, once I find it.
Have a good break Rossco and enjoy christmas new year camping with your family.
Macca
A search in ebay (USA no .com .au etc) I think a US gallon is 3.7 litres
http://www.ebay.com/sch/i.html?_nkw=...=p5197.c0.m627
Hope that works, might take some trolling to find something suitable and reasonable frieght
Dads 10gallon cost $94.52 from www.keithsrvsurplus.com delivered
Macca