Like Randy mentions, you really need a larger trailer than a 5'x8' for many reasons.
I would not go with a single axle trailer as the weight of the hot water skid will be from 700 to 900 pounds by itself without fuel in the tanks and water in the coil, that will add up to another 200 pounds depending on the size of the coil and fuel tanks.
The trailer itself weighs so you deduct that from the total capacity and you will know what you can put on/in it.
That water tank when full with the hot water skid and other tools/supplies hitting a pothole can easily bend an axle on a single axle trailer, I would seriously go with a tandem axle trailer, besides the added strength and capacity, it is harder to fishtail the trailer when backing up.
There have been many threads about this in the past and some great advice, use the search button to read what some of the professionals have advised others to do, it will save you a lot of time and aggravation in the long run.
The best professional hot water skids are Hotsy, Landa, Alkota and Largo from what I have learned from the pro's out there that use them day and nite, then you have the Aaladin, Hydrotek and the others that are available with many names, dealers putting their names on generic machines, etc........ The first four are more expensive than most others out there but are very well put together and have lasted with guys out there running them many hours a day for many years.
Good luck.