Dry weight plus weight of:
Batteries
Propane
Fresh water in tank
Fresh water in water heater
Water in sewer tanks
Food
Clothes
Gear
What ever toy goes in the toy hauler.
You will likely be really close to that 12,500 pounds.
Vehicle tow ratings are given with empty tow vehicles. Every pound of passengers, pets, tool box, tools, truck accessories, cooler, firewood, etc. you load into your truck deducts from the tow rating.
Most vehicles, particularly 1500 series trucks, will run out of payload capacity long before they reach their tow rating when towing a hard sided RV.
Tow ratings are rather generic; there are many types of trailers: flat bed, horse, equipment, dump, boat, etc. I would say that by far a hard sided RV is the most difficult to tow based on it's huge frontal drag, large side sail area, high percentage of tongue weight, and inability to change much of the tongue weight.
A truck that may handle an 8,000 pound boat just fine may be a struggling handful with a 7,000 pound RV.
If you travel alone (light) and are careful you might be able to make it work. I think the experience would be much more enjoyable with a smaller trailer or more capable truck.