New to towing a caravan

HI there, just bought our first van at the ripe young age of 51, looking forward to it after a horrible camping trip with wind and rain this (so called) summer.

Its a 2006 Bailey Pageant series 5 Burgundy and is a lovely van. HOWEVER, i have a problem already. My break lights wont stay on. I hook up the van and the side lights work, indicators work, fog light works, but the brake lights will come on for a second, and then go out, leaving just the side lights. If i do this a few times the same happens, but after 4 or 5 times, it then just doesnt light up at all.

Any help would be greatly appreicated, and apologies if this is not in the right location on the site, but looking for advice on what the problem could be. I have looked at the plug and all appears to be in order.

Regards

Graham and Fiona