No offense, but I think you are wasting your time,, and money. Consider the R-value of your sound deadening,, and then try to figure out how to actually get it to stick on all sides. It really is a matter of figuring the BTU loss from the tank given any wind drawing those BTUs off and you will soon realize that it is going to be really hard to keep the water tank from freezing. And then you have the problem of the water lines that are under the van. RT does it's plumbing as cheap as possible, with the bulk of it under the van, including the suction line from the tank to the pump, the hot and cold to and from the water heater and to the kitchen sink. Once again, even with proper insulation, these are all exposed to the air and even at say 25F, driving 60 mph will freeze them. ( I know,, I have done it!)
My indoor water tank change, eliminates the under the vehicle water lines (or in the case of the kitchen sink and suction line) valves them off so that they are dry even while I have running water in the toilet, shower and hot water heater. I cover the water heater while driving with a foam cover replacing the grill when it is cold. The water heater it self is insulated, so if you heat it before you drive, shut it off, cover the face (including the PRV and drain/anode as they are giant heat sinks!) You can drive all day and the water heater won't freeze.
The grey and black tanks are not a problem by adding a good quantity of potable anti freeze every time you drain some into the tank. You have to keep the concentration fairly high to prevent them from freezing, but even mixed with water, the grey and black tanks will only slush up, and will take that without damage. The macerator pump itself should have a ration of pure anti freeze poured in after draining so that the pump housing won't freeze,, and expensive repair.
Good luck,
Icarus