10A is more than enough for 10 x 21w bulbs (2 brakes on car, 2 brakes on c'van and 6 indicators)
could be a dodgy bulb or damp / wet / short circuit in the trailing socket / c'van light fittings
try doing the same without the c'van lead attached, if fuse ok, you know your cars ok
then try do some basic fault finding, check the sockets are ok, and also take the lens covers off the c'van and look for water ingress