Rest areas

SimonAndAnnie

We are new to caravaning and on our trips out so far we have found it difficult to find places to park up for a rest en route and to give the dog a stretch of the legs.  Obviously, motorways have large service stations with defined spaces for caravans. 
But it is a bit trickier on "A" roads.  Lay-bys are not always big enough and can even one truck can make it impossible to get in.  Has anybody seen any kind of route planner that shows parking or rest areas suitable for caravans?