No idea about suppliers in the US, but before contacting any spring company you will need to determine some parameters first.
I will start by saying that there is no "one size fits all" solution here. You need to have an idea of the weight of your camper body, how much separation you want to allow between the chassis and subframe when it flexes and the basic dimensions of the springs you want to use.
With that information at hand you can specify the overall spring length, maximum outside diameter or minimum internal diameter, wire size, number of active coils and spring rate.