Afraid you're in fopr a disappontment John - such things seem to be rarer than rocking horse droppings! Seriously, I spent ages some time ago trying to source exactly what you are looking fo and drew a complete blank,. Depending what it is you want to time control if the current draw is low enough you could install a 12v to 240 v invertor and then use a standard 240v timer.
Even though you don't answer the questions posed in earlier replies to your question, it occurs to me that alternative energy suppliers may be able to help. You know, those people who help the solar/wind self-sufficiency brigade. In the UK, an outfit such as Wind and Sun (www.windandsun.demon.co.uk) should be able to supply a 12 volt timeswitch.
Thanks for all the ideas folks .... I won.t go into details just a daft idea for accessing my boat when I am not there in person and maybe switching on a fan heater now and then to stop the mildew.