So, how cold can we reasonably expect to camp without disaster striking? Would high 20's/low 30's be OK or is anything close to freezing too risky?
There is no generic answer to your question. To many varibles...
wind
camper insulating value
tanks locations
under belly enclosed
3 season vs 4 season
not all 4 season campers have the same insulating values or same type of heated tanks.
etc
There all different.
We livid in a so called 4 season 27' 5er for one winter while the house was being built. Underbelly was not enclosed.
It had two grey and one black tank exposed. We had a heat blanket on the bottoms and then had them encapsulated with expanding foam.
Heat tape with a thermostat for the fresh water hose.
The unit was permenant with mobile home skirting all around so no issues with zero temps and 40 mph winds.....other than severe interior sweating on all the outside walls.
Two dehumidifiers cured that issue fast.