The answer is....................maybe. We take the ferry from La Paz every year. We have a short bed pick-up with a 6-foot pop-up camper on the back. This year we are a little longer than 6 meters because we have a double cab in the pick-up. Every year they want to class us as a mini-motorhome and charge us nearly $1000 one way for the trip to Mazatlan. So far my husband has been able to convince them to create another category for us so we pay about half that.
My suggestion: Go to the downtown office for the ferries in La Paz. They seem more willing to think outside the rule book there. Charges are really high for any "house on wheels." If they want to classify you as a motorhome ask to see the supervisor. Be nice, be polite, plead your case, tell them it is so much smaller than a motorhome, etc. etc. And keep your fingers crossed. If you can't cut a deal, there is a cargo ferry that goes as often as the passenger ferry. You can book passage on that but they have no rooms, only salon seating. Some folks have been able to arrange to park their motorhomes on the upper decks in the fresh air and then just live in them for the crossing. Could be a lot of fun IF you can park on the upper decks.
Good luck. And don't mention us 🙂