Leaking roof lights and windows

i descovered i have a leaking roof light in the kitchen on my motorhome and a leaking back window , i took it to a repair place and they did a damp check (after i did one ) all was fine But he said £100 for sealant ..8 hours work..@£45. /hour for the window and roof light vent window out ,it could also be the fibre glass and aluminium section joint  so worse case  £460 plus vat  to me thats £575. if i am not mistaken . he advised i get it done now rather than later as a chap had left it too long and they had to replace the back wall at a cost of £6000. .so if you get a leak sort it quick