Where can I find Used Tow vehicles and Equipment for Sale?

craigslist.org

https://www.autotrader.com/

OP, without more information from you, all we can do is throw out basic suggestions.
I don't know of anything exists, in the way you described it, other than standard places like AutoTrader or Craigslist.

It would help if we knew where you are.

I bought my F-450 from a used truck dealer in DeLand, FL. They had over 100 light and medium duty trucks on the lot to choose from when I was there about a year ago. The name of the company was something like Florida Truck Sales if I remember correctly.

I found my truck on cars.com a couple years ago. Bought it from an individual.

Do you know what you will be towing? Popup, TT, Fifth Wheel?
You don't want too little tow vehicle.

There is a RV Forum l Click Here for Escapees RV Club they have a market place for all kinds of things that you might like.

Easy signup just like here.

There are millions of used vehicles for sale.

Tow vehicle? Used car lot comes to mind...

Craigslist is a good source. RV dealers some times have used equipment as well.