You are on the right track with a CPU heatsink. Without the fan it's probably capable of 5-10 watts on its own. With the fan, you're good for 45-60 watts. Most standard CPU fans run off 12 volts, so you don't need to reduce the RV voltage, they'll be happy with 13-15 volts from an RV.
Regarding dimming, its best practice to have the LED driver (dc to dc converter) located as close as possible to the LED. Dimming can be controlled by a distant remote dimmer if desired.
There are a few ways to do this:
1) Using a 120 volt AC LED driver that supports AC dimming using an off the shelf standard dimmer switch (this is the way many led-retrofit fixtures are setup)
2) Using a DC LED driver that supports 10-volt dimming using an off the shelf standard low-voltage dimming control (more of a do it yourself project)
3) Using a pulse-width modulated (PWM) signal to turn the LED driver output on and off so fast the LED appears to dim to the human eye (definitely a do it yourself project, but not that hard really)
Low power LED fixtures (well under 1 watt) can also be dimmed using a "linear dimmer" which basically reduces the voltage going into the lamp. This method is not recommended for high power LEDs because of high losses incurred in dropping the voltage at significant amperage.
I'm more than happy to go into detail on any or all of this if you'd like.
