While there is some added risk with the propane tanks open during travel, due to the pressurized lines running through the rigs, in a severe crash you still have the propane tanks that can be severed and dump propane. If you run with open tanks, for the frig, then there is no real additional risk with the furnace. turning off BOTH is a good idea during fueling, but still a pretty remote risk.
In a class A you've got the fuel (diesel or gas) also to worry about.
An MH is a traveling bomb anyway, and so is a car, so you takes yer chances.
Some folks get pretty obsessive about these issues, but it really is a dangerous sport driving down these roads in these huge boxes, but we do it anyway.
2007 Chevy 2500HD Classic LBZ Duramax/Allison (Stock)
2005 Bigfoot 1500C8.2 AWESOME