Mixing wires in a narrow chase.

I would not expect much if any interference. The solar charge controller operates at a much lower switching frequency than cell phone signaling, by at least a few orders of magnitude. Putting a capacitor (a type with low impedance, preferably) across the solar panel inputs to the solar controller would help control any high frequency noise, as likely would putting a ferrite core over each of the wires near the controller. But I rather doubt either of those is necessary; if it were, you'd have poor cell phone reception in the vicinity of the RV already.