Life's A Series Of Tradeoffs...
I keep up on all things Alt Energy/Solar, and this one is a lil tough. The adjustable Controllers out there, used in Big Rig RVs or Home Solar installs, work off of Solar Panels. Somewhat pricy, but they're rugged.
I used a 400 Watt Inverter from Amazon while SUV Boondocking around Lake Powell AZ/UT. Good Reviews, and it held up fine. It could supply 115 VAC on the cheap to power Line Voltage, adjustable Charge Controllers. Not your ideal; but one answer...
[ame="http://www.amazon.com/Cobra-400-Watt-12-Volt-120-Volt-Inverter/dp/B001RNOHBC/ref=sr_1_1?ie=UTF8&qid=1297867797&sr=8-1"]Cobra 400 Watt Inverter[/ame]
Some Geek has created what you need; I'm not aware of it, unfortunately. The difficulty is in controlling a 2nd, lower voltage while current varies as the Battery charges.
I've pretty much settled on Dekas for my used lil TT just acquired that I intend to mod technically for Boondocking. I'll likely spring for 2 ~ +6 VDC Deka Golf Cart Batteries series-connected to net 'lots' of AH capacity [~250 AH]. I'm also looking at AGMs, due to being able to slam >10% of the recommended AH capacity into them while charging [the 'usual' guideline Battery charge rate]. Golf Cart Batteries tolerate deep discharges >50% of capacity. They tolerate abuse, needing only occasional care & feeding to net very long service lives.
If all considerations permit, a 2nd Alternator mod'ded to output only the voltage you need
might be easier to pull off technically than trying to 'adjust down' and maintain the lower voltage req'd. Another thought is that a second mod'ded Regulator working off 1 Alternator might be added, and used through a 2nd Battery Switch setup. Alternator outputs 'spike' when switching is done while running, so one has to be careful about charging 2nd Batteries. A ~$100- 'box' from RV Shops prevents that problem.
Some App Engineer at Deka might have a ready solution for you. Look around at Alt. Energy/Solar Sites; there's some really creative Geeks out there solving these problems.
A pal also tweaking a used 15' TT bought a Cig Lighter-based Voltage Monitor for ~$12- I saw at Wally World. The readout is 13.8; i.e., to tenths-of-a-Volt. He had it checked against calibrated Voltmeters where he works, and it was accurate to within 0.1 Volt. My style would be to bag the Cig Plug and panel mount it.
I got a 'free', unused, quiet 3kW Kipor [clean] Sine Wave output Genny with this Trailer, so I put on hold my 'Plan A' for PV Panels, or to build this little Genny:
Lawn Mower Engine Genny Project