An "open jaw" is basically a "V" shaped itinerary, A to B, C to A, or A to B to C. Technically, to be a true "open jaw" to get the best airfares, the distance of the "open" segment can't be the longest of the three legs; for example a route like New York to London, then Miami to New York isn't a "legal" open-jaw, and will likely be priced by the airlines as two one-way flights.

In your case you're talking about a series of one-way flights, which will be priced accordingly by the airlines.

That's not necessarily a terrible thing - many airlines are increasingly pricing flights on a strict one-way basis anyway, with round trips being just 2x the one-way fare.

You might look at 3 round trip tickets per person just to see how it works out - Florida to Columbus round trip, then Columbus to MSP round trip, then MSP-MIS round trip. Or run your itinerary through the "multi-city" options on sites like Expedia or individual airlines to see how the numbers look.

What exactly you want to do is not quite clear from your post. You have a trailer that you are going to drive to Montana, and leave it there? Is that right?

And then, what?

It would seem to me that you would then want to fly from Montana back to Florida with stops in between, but your post says no, you are flying FROM Florida.

???

I agree with Gardlyloo. Just check on the one-way flights, even when you want to do a round trip. Lately you can find low-priced one-way fares that are 50% of a round-trip ticket. And, if you search one-way fares you may find one airline cheaper in one direction while another is cheaper in the other. Sometimes these combos show up on expedia and travelocity and the like, but sometimes not. And sometimes those sites add a fee of their own that you don't pay by booking directly with the airline. I like to start at Kayak.com.

Not sure I understand correctly- Leaving truck and trailer in Montana and then want to see daughters in Minneapolis and Columbus, OH on the way back to Florida. Is that right?
If so, You could ride the Empire Builder from Montana to Minneapolis. Visit daughter #1 then get back on the Empire Builder to Chicago and rent a car there to visit Ohio.

I think they're stationing a trailer and truck in MT for vacations/summers; the rest of the time they live in Florida. They're not keen on driving all the way to MT for their (annual? frequent) holidays, so they'll just store the rig there. They want to stop en route at CMH and MSP, both directions.

Gardyloo is right - we want to store the trailor and truck in Montana, fly out in June, use the rig for trips and as a home and store at the end of sept and fly from MT to Fl with stops in MSP and CMH to see the kids!

Well, I think you would book an open jaw from Florida to Montana in June, with a return from CMH to Florida. Then you'd have to book one way tickets from Montana to MSP and MSP to CMH.

As for prices, they change all the time, but if you were doing this next month the whole shebang would be about $900 per person. The one-way from Missoula to MSP seems particularly expensive, around $320pp. When flying in and out of Columbus, also check rates from nearby Dayton as they can be quite a bit lower at times.