The GVWR of any chassis, is the max amount of weight the vehicle can carry. It's everything, gas, water, people, stuff. Going over the GVWR will overload the chassis, suspension, and tires. Not a good thing to do, and not a safe thing to do.
The difference between the empty weight of your RV and the GVWR is the Occupant & Cargo Carrying Capacity (OCCC) of the RV, in other words the weight of people and stuff you can carry. You should try to determine this number. Load up your RV with full gas, full water, full propane and get your RV weighed. Best is four corner weighs, or at most truck stops you can get the axles weighed. Subtract this weight from your GVWR and you will determine your OCCC. The answer to your question then is NO, you can't assume you can load up 1200 pounds and you will be OK. Best wishes for safe travels.