Well, for one I have a pair of Freeze switch plugs, These are thermostatically controlled outlet adapters which turn on at 35 to 40 degrees... Plug some 100 watt light bulbs into them (put the bulbs in the compartments under the tanks) and you have an istant heater system.. Course it burns up electricty, but that is the current plan (I may use one of those plugs in the entainterment compartment to prevent freezing of some of the electronics I want to work year round, even when the rig is parked,,,, IE: Sat TV stuff) I may also run a 100' coax line so I can watch the rig's Sat TV while parked at home)
That is the big one... Insulation (Adding it) can help, you have to know where to add though. Skirting may also help, however you need a skirt that is easily removed... I would suggest getting some tarp (possibly rip-stop nylon) and cutting it in strips about 2x distance between the bottom of your compartments and ground... Along the "Bottom" you are going to hem the fabric but you will sew somethign like a sand sock or a chain into the fabric (Weight to keep it on the ground... Needs to be flexable cause when have you seen perfectly flat ground) top end gets a dowel rod Place dowel in the storage compartment and close the door to skirt the rig, You will need to get inventive around wheel wells and/or front/rear of the rig however.. This keeps cold air from blowing under the rig and can actually help.... (Method 2, for long term parking, bales of straw under the rig.. Again use something soft and flexable however to finsih the skirt cause you don't want to bring the rig down on the straw (less you bring frame rales down on it) due to the fact that you might damage your rig.
I like the tarp idea best though, Custom cut a tarp for the job... I may hae to build one of those
That and the light bulbs or other tank heater options
NOTE problem with tank heaters... They heat the tank, NOT the water lines that run around the rig