Typically those switches are only used for one item such as ignition or lights or whatever. Master disconnects are used to supply power from the battery to multiple accessories. Since you're running propane you have a fewless items, but typically these would include electric fan, fuel pump, ignition, lights, etc.
I guess what I'm getting at is that switch you show would be good for killing your ignition which should shut the truck off, but it would leave all other accessories powered up which could be an issue.