I missed OC's question earlier... OK cambering a beam is giving it a crown (positive camber) by basically prestressing the beam so that deflection under load basically flattens out the beam versus inducing a sag (negative camber) in it. Camber actually does nothing at all for structural capacity. It's basically an aesthetic consideration that provides engineering efficiency because you can use a smaller cambered member versus using a heavy straight one to achieve the same deflection limit.
Now as a structural engineer, you are taught you don't camber short beams (less than ~25 foot), shallow sections (web less than ~12" depth), beams used in moment frames, and beams that are cantilevered. Mechanical engineers, where crossing the domain of structurals (yeah, I know), challenge these rules somewhat because they depart from AISC standards and are working with fabricated beams (like trailer frames).
As the OP raises concern, and why structurals avoid camber in cantilevers is because of a spring-like effect. It's not at all the action of a leaf spring where spring steel behavior is different (and can take the cyclic deflection), but is the eccentricity once loaded that causes a greater reaction at the rigid support that over-stresses - liken to why the really, really large fellow shouldn't use the diving board at the pool.
With a trailer frame, when hitched, each frame rail can be treated a simply support beam with the pin corresponding to the coupled hitch and the roller corresponding to the axles. The length of span from axle to bumper gets interesting in the case of a toy hauler where the greatest load could actually occur versus in the cabin (and the span between coupler and axle where typically cambered). So, in the case of a toy hauler garage frame, things get interesting if camber is induced, CCC is not stellar and the owner goes for using all of it (and often more) to stuff the garage.
Lippert does camber several of the longer camper frames. They have to because of the shallow beams they are using for frame rails to conserve weight and clearance. However, I wouldn't expect it for a toy hauler frame.