Reducing Panel Power for PWM

I have a special project to charge 2 batteries from 1 panel. One is a motorhome chassis battery, and the other is a tractor battery. Both are 12v, but different capacities.

I already have a panel to use for this. The specs are as follows:
Power: 255 watts
Vmp: 30.77 V
Voc: 37.83 V
Imp: 8.37 A
Isc: 8.90 A

I am looking at the Morningstar SunSaver Duo (http://www.morningstarcorp.com/products/sunsaver-duo/). The specs say it needs a source of 18v-25v and 25amps max. The panel I have is clearly over those specs.

The question is, can I install something like a zener diode to reduce the panel voltage to what the PWM needs? Something like a 10w and 24v zener? From the quick reading about them I have done, I think I need a resistor as well for backflow current, but this is not my expertise.

Or would doing something as simple as covering the panel with shade cloth work and not harm the panel? This is basically just to keep these batteries on float charge.

If I am way off, I am gladly open to suggestions on how to make this panel work with either this PWM or another one that charges 2 separate batteries from 1 panel.