Hi avidbowhunter,
The Heartlandrvs.com website shows current Sundance 5'ers with a GVWR of a bit under 14,000 pounds. So there's pretty good odds that yours is around that same max weight rating. If the weight sticker has been damaged, you might be able to get a replacement from Heartland. Call Heartland Customer Service at 877-262-8032 and have your VIN # handy. They can probably tell you the actual GVWR for your model and whether a replacement label can be obtained.
With respect to your truck, there are two issues to consider. You've already looked at how much weight the truck can tow. If it's rated for 12,500 vs 14,000 for the trailer, you're overloaded a bit. But you might want to check the numbers on your truck again. Make sure you're looking at the 5th wheel towing number for your configuration.
The other issue is vertical load on the rear axle. Current Sundance models have pin weight from 1860 to 2145 pounds - empty. After trailer options and your gear are added, the actual pin weight could be a few hundred pounds heavier. You need to compare that to the payload spec on your truck. If the payload spec if 2,200 for example, by the time you put 2,200 pounds of trailer pin weight on the hitch, you've nothing left for weight of the actual hitch, tools, bed liner, and passengers.
To research this further, you should visit
Fifth Wheel Safe Towing, a website created by another Heartland owner.