Hi,
What model year is your C-951? We can help look up the tank size if it is listed in the Sunline sales brochure, but knowing the model year helps hone on on yours. They may have changed sizes over the years on the same model.
For example, a 1988 C-951 has a 23-gallon fresh "system," of which 6 gallons are in the water heater and maybe a gallon or two in the piping. The tank may only be a 15-gallon tank. Once the water pump loses prime, the 6 gallons in the heater and approximately 2 gallons in the piping are stuck there. When the fresh tank is dry, and the pump loses its prime, that is all that can be used.
The 1988 C951 has a 13-gallon grey tank and a 10-gallon black tank.
But that is for a 1988 C-951; the tanks may have changed in your model year.
Glad your trip went well. And yes, regardless of year, the grey tank is going to be small on the truck camper.
John