Hi Yellow,
I think Heartland only recently moved to support 2" receivers. The Heartland website shows a 2" receiver for 2020 model year Bighorns, but the 3160EL is not one of the 2020 floor plans shown. You may have an earlier model year that doesn't support a 2" receiver. It's possible that there may have been changes to the Lippert frame to allow newer trailers to support a 2" receiver. If that's the case, your Bighorn may not have those changes.
To be sure what's supported, you can call Heartland Customer Service at 877-262-8032 / 574-262-8030. Have your VIN # ready.