Hi there Al, the 1st bit of advice I could give you is to make sure that your tank is baffled and is secured well to your trailer. Also make sure that the trailer is rated for the weight of all your stuff, your tank will weigh nearly 700Kgs when full, and your 25L di vessel will also weigh a fair bit and thats without mention of a battery. If you go for a leisure battery they weigh a ton, or if you go for a sealed lead acid battery which has smaller capacity, they also weigh less.
If you wanna see my diy trailer mount here's the link to the pics,
http://www.cleanitup.co.uk/smf/index.php?topic=19436.0Dont forget that you will need a hose and reel, dont be tempted to go for a cheap plastic and aluminium one, been there, done that, then gone and got a decent metal one from Gardiners. The difference is tremendous, much easier and quicker.
You also will want to have your RO filling up some water butts, ideally if you have the space for it get a 1000L IBC so that you'll never run out. You also might consider getting a 300GPD or even a 450GPD RO, and get a pumped one if your mains tap pressure is below 45psi.
Hope this helps,
Tim