Misleading avertising/ignorant sales people

As you already know from your post actual towing capacity is dependant on several things. You are looking at spending a chunk of change to get something that can actually tow over 5k#. You need to know and understand how to do this yourself (which you have already demonistrated) when investing in something that expensive. Don't expect dealers to even understand some of this stuff unfortunitely

BTW there is no regular class C that can do over 5k# towing. You will need a super C or a larger (read DP) class A to do that.