There are many alternatives to consider.
First of all you may wish to keep the Edison bases. There are 12 volt
LEDs available with a screw base. Here is one example:
DR. LED Bulb-led at West Marine
If you do decide to switch to an 1156 style lamp then there are converters available so you can retain the original socket and mounting hardware
ANCOR MARINE Base at West Marine
But I would not opt for an LED conversion with those fixtures. LEDs do not perform well in fixtures that are not engineered for them, and overall there are still many problems with lamp life, light quality, intensity, and efficiency. Instead, consider fluorescent lamps with a spiral tube, which are available in 12 volt
configurations with an Edison base from many sources. Here is one web site, there are many others:
12 volt DC compact fluorescent lights, 3 to 30 watts in spiral, double U tube, or bulbed, soft white and daylight, quantity discounts.
In practice with the kind of fixtures you have you won't get much more useful light per amp from LEDs than with the compact fluorescents.
Finally if you do replace fixtures you might consider the Taylorbrite fluorescent lights that are used in current production Airstreams. I have them and they're great, although I had to replace the tubes with ones with better color. These take up less headroom than the old fixtures - important if you're tall - and produce a lot of light.