The prototype of the shutdown controller is finish and all work perfect
It can control a current around 30 Amp.
1. Tank battery automatic switch to tank battery when main power loos, this controlled by to relay
2. to control the main power to the dc-dc converter and power on and off the pc, I use a PIC controller coming from PICAXE and it only cost 2 $ and very easy to program.
I use standard car relay with homemade connector, maybe not nice but it wrok
The next step is to make the final layout with 2-3 more function where I need a bigger PIC with more I/O pin.