Here's how you do it:
1. Open up the cavity where the tanks are and assess the situation.
2. Determine how much room there is for insulation and whether the insulation will impede the functionality or serviceability of the camper.
3. Decide which insulating material will work best in the application. Multiple materials, such as spray foam (Great Stuff), fiberglass, and foam board may be required.
4. Install the insulating material as determined by steps 1-3.
Be aware that insulation does not prevent freezing. It only slows the process. If you put a camper in 0 degree weather long enough with no heat input, the tanks will still freeze no matter how much insulation you add.
It is likely that there will not be enough room in the basement to add any appreciable amount of insulation, so any effort will be wasted time and money.
Putting 10-ply tires on half ton trucks since aught-four.