Of course, as stated earlier, premiums are all over the map depending on where you live, your desired coverage, and the value of your TM. However, in my experience, with all those being equal, the least expensive way to get coverage is simply as an add-on to your existing auto policy. Our premium is about $100/year that way, and always has been around that since we bought our TM when it was 7 years old.
Dave
Man. All of our insurance is with Allstate, so that is who I called first--thinking as you said it would be least expensive to add it to our existing auto policy. It was going to be $500 a year! I think it is because we are insuring 4 drivers on that policy--two of whom are teens. They don't care that my daughters won't be towing the camper. I (((think))) that is why the quote was so high with them. Progressive is who I went with. Still seems a little high, but they asked how many years of towing experience we have--which is none--so there's that.