hi where is the best place to fitting the battery in hobby prestige 635uk
I'm going to copy this answer to the Technical area, where you are more likely to get assistance.
I am not familiar with the details of your caravan so I can't offer a precise answer for you, but there are some general guidelines that might help.
For safety the battery must be secured inside a sealed box which has air vents to the outside only to allow any gas caused by charging or any spillage of battery fluids to be kept away from the occupation space. There are several proprietary makes of battery box which are designed to be accessed through the side wall of the caravan.
Because of the weight of a battery it should located as close as possible to the caravans main axle.
The battery's 12V positive lead should have a 20A fuse incorporated as close to the battery as possible.