I'm not going to bother to use a DMM or analog volt meter because this is not a subject I need to prove or disprove since it is something all of us who use winches already know. And I started building/designing electrical and electronic circuits back in the early 60's so that's over 50 years of knowing what is up in this subject. I also taught electronics in the military and commercial industry so it's not a subject I am unfamiliar with.
Go install a winch on your rig and tell me what happens when you start winching heavy loads using normal winching cycles/times. You'll quickly see your concerns are not valid. The vast majority of us who have and use winches have a single battery and the factory alternator. That should be a pretty good indication you're unnecessarily worried.
Since few of us with winches (do you even own one?) have problems with our single batteries and stock alternators, I'll just have to say it's not a problem. If it were, we'd all be installing dual batteries and huge honkin' alternators... and we're not.