http://www.prestolite.com/pgs_produc...&submit=Search
Just for the halibut they did have some infor that it may still be available in the UK operation.
May or may not be worthwhile to contact them.
I'm assuming you've run into a wall through Onan also.
Not knowing the setup makes suggestions difficult but if it is a hangon, rather than built into another piece, it may be possible to adapt another solenoid from a starter motor or similar.
For the hold in function you would need to figure out a resistor that could drop the voltage, maybe to 8.5 at the solenoid and withstand the heat.
Have you talked to a starter motor,or alternator repair shop. They may be able to help. Give them the p/n you have. Although prestolkite may not be helpfull, there are other mfgrs. and maybe there is an alternate that way.
If all that comes to naught, can you (would you consider) rigging a push/pull cable like is sometimes used for throttle controls.