Floyd,
As I mentioned in my earlier post, all MPPT controllers are PWM DC to DC converters. The RFI from the controller could be common mode or differential mode. The common mode problem is easier to solve so I recommend that you start there to see if it gives you relief.
For common mode RFI in the frequency range you mentioned, you should try using type 43 or 73 ferrite material in a full toroid (not split core) form. In general, the more turns of the wire or cable that you can wind on the toroid, the better. So consider using a 240 (2.40 inch) form. Wind the DC input leads (plus and minus) from the solar panels on one form, wind the DC to battery leads (plus and minus) on another, and wind the load leads (plus and minus) on a third. Six to eight turns is a good start, more if it can fit in the core. Keep the leads parallel to one another as you wind - avoid twists in the leads.
For differential mode interference, you need to deploy a brute force type filter. This will look like a low pass filter designed for the currents and voltages involved in your solar system. You should consider one for the solar panel connections, one for the battery connections, and one for the load connections. You may be able to use a suitably rated AC power line filter for this purpose. Take a look at this schematic for an idea of something you could scale up to your current requirements (from ARRL publication
Setting Up Your Station, DeMaw, QST July 1984). Increasing the gauge of the wire and using sound construction techniques may be all that is required for your situation. Do take note of the grounded, metal enclosure.

I do not recommend building this filter as shown for an AC power line application. It presents a shock hazard if the ground is not connected. That is not an issue for circuits up to 48 volts such as typical solar applications although the ground is still required to be effective.
You can substantially improve the performance (and ease of building) of this filter if you have access to suitable feedthrough capacitors. Mount these through the walls of the enclosure as the input and output connection points and connect the inductors between each pair. See the API Technologies PSM4-103Z-20B series from Mouser (
http://www.mouser.com/ProductDetail/API-Technologies-Spectrum-Control/PSM4-103Z-20B) as a 20 Amp/200 VDC example. You can also add turns to the inductors if you wish since you are operating in a DC environment.
The common mode and differential mode filters should be installed as close as practical to your MPPT controller in order to squelch the RFI before it is radiated or conducted elsewhere. With a little clever engineering, you could combine the common mode and differential filters into a common assembly.
If you have the remote metering feature, add common mode filtering to this connection as well. Unless you are aware of the baud rates involved for the communications, I would avoid the use of a differential brute force filter since this could impede proper communications.
- Glenn W9IQ