Insulating an external tank just delays the inevitable unless you can provide some sort of external heating. Pipes are particularly vulnerable even when insulated, especially if nights are well below freezing and the days are chilly because the sun never gets to the tanks anyway. Grey tanks are best dealt with by keeping them empty with the drain valve open and a bucket underneath. Freshwater tanks inside are usually OK provided the heater is going, although as others have said, you need to keep cupboards open at night to allow the heat to get in, especially if there are pipes which touch uninsulated parts of the wall - steel bits that go through the wall or floor, or fibreglass wheel arches and the like. You could also - if you have a blown air heating system - add some skinny ducts and direct the air around tanks and into underfloor spaces.
If you are going to be parked up for a while, it would pay to set up some sort of a skirt around the vehicle to stop the wind blowing underneath. You can buy external or internal elements for tanks and heat trace for pipes but these aren't really useful unless you are on hookups or have the engine running.
PS if you are using Butane, unless your gas locker is heated a bit, you may find you have no gas when the weather is below zero. Use propane to save hassles.
If your question relates to what to do in between winter trips, you only have two real choices
First is to have it on EHU and run a thermostatically controlled heater to stop everything from freezing - and even that may not stop tanks or pipes from freezing. With no heating and temperatures below freezing, everything WILL freeze regardless of insulation
Second is to set up the pipework so it can be easily drained in between trips - and don't forget the water heater
and
battery.