Mark, there is not a specific programme for the TomTom for caravan routes. There used to be one on the AA website, but even this has been discontinued.
Having been in the exact same position as yourself last summer, Grand Espace and 7.2 mtr van on the back down a VERY narrow lane with hedges brushing each side of the van I know the nightmare.
However, if you get TomTom to give you the route to the site, then using the directions in the handbook, programme it to go via the suggested route from the nearest main road.
As with any nav system, it is an Aid, nothing more, and relies on common sense when things get difficult. Dont forget you can get TomTom to give alternative routes at the press of a button, or having ignored a direction, it will recalculate a new route in seconds.