Insulation, mass and heat
One could get some water heater wrap to insulate the batery box, this would reduce heat loss from the batteries.
Next, the batteries are very heavy, it takes a lot of time and temperature differential to actually drop the temperature, so insulation will slow it down.
Next is heat, depending on the use of the MH, one could route a water line from the engine heat into the area with the batteries, a single turn or 2 of copper line secured to the wall inside the insulation would add heat, a ball valve would shut it off durring summer, use a return line from the heating system and use a "T", if no "T" then depending on your system the dash control may be OK.
So while driving it warms the batteries up, then the isulation helps keep the heat in so it is not as bad.
Our coach has a duct from the heater into the chamber that has the water tank to keep it warm, you could also do this, find a duct close to the battery box from the heater that you will use, tap off and rout it into the insulated battery box.
Small 2 inch or so is all that should be needed, but you can determine this epending on your heat loss.
If AC is not available then battery power or mass will be needed, I think you best bet is insulation.