I don't see any reason why you need to specifically reduce the voltage of the solar panel.
Sure, it will be pulled down below its most efficient operating point, but so what ?
All you need to do is disconnect it when the battery voltage reaches some preset maximum, say 14.2 volts, and re connect it when the battery voltage falls below some preset minimum, maybe 12.6 volts.
The battery will very quickly come up to 14.2, then very slowly fall back to its resting voltage. A resting voltage of 12.6 is considered fully charged, and it should hold that voltage for a long time if the battery is good.
The charge, rest, charge, rest cycle is MUCH better for the battery than floating it at a constant continuous 13.8 volts.
I have been doing this for almost two years on a vehicle I very rarely drive. Power comes from a 15v 2A wall pack through a 12v 18w filament lamp to limit current. you probably will not need a current limit with a solar panel, but that is up to you.
After a while the charge cycle gets shorter and shorter, and the rest periods get longer and longer. Its very good for the battery and the battery has used zero water over two years. In fact the battery seems to be in much better condition than it was two years ago.
You can buy a programmable voltmeter that will do all of that. It will read battery voltage to four digits, say 12.34v and there are two independent one amp relays that you can program for exact pull in voltage, and exact drop out voltage in 10mV steps. That is what I am using and its probably all you need except maybe a more grunty external relay or possibly a lamp to limit charging current.
http://www.lightobject.com/Programmable-4-Digit-Red-LED-ACDC-Volt-Meter-with-Dual-Control-Good-for-HHO-System-P408.aspxJust connect up a few wires, program the voltmeter for whatever voltages you like, and the job is finished. An occasional glance at the battery voltage will tell you exactly how the battery is doing.