Shaun, it's not the boiling of the water that causes the breaker to trip, it's a combination of the pump, 2 vac motors and a heater that make it trip, especially the surge of power at start up.
We clean LOTS of caravans every week now with the TM, (14 this week alone), we previously did a few of them with the porty however you had to be very careful on some sites as they only had a 5amp or 10 amp supplies.
I very rarely used the heater and never when I was actually running the machine. I also often blanked off one vac motor and just ran on one.
I always started things off one at a time to minimise the sudden surge of current to the breaker, ie: start the pump, wait, start first vac, wait, start second vac.
Easiest way is to buy a Prowler!!!!