I think if you are operating "in furtherance of a commercial venture" you need to be DOT registered. If your farm is a commercial venture and you're furthering that by advertising it on the trailer, I'd guess you need DOT registration. It's not expensive, just the DOT part. Now, I'm pretty sure you can't legally use RV plates on a commercial vehicle. So, that means truck plates. Unless you go interstate. That puts you into an Apportioned plate I think. Which also puts you into IFTA (Interstate Fuel Tax Aggreement) requirements. More paperwork and expense. Then, you'll probably have to pay more for insurance since it's a commercial vehicle, not an RV. I think this is all if you want to assure you're operating fully legal, which I'm probably missing other things. Just putting some "private rv, not for hire" or some such stickers won't cut it with a stickler of a DOT Commercial Vehicle Enforcement Officer. I know a couple of them and they say those are actually a magnet for them telling them that you are more than likely commercial and they're gonna go after you.
'03 Freightliner FL112, 295" wheel base, with '03 United Specialties 26' living quarters, single screw, Cat C12 430 h/p 1650 torque, Eaton 10speed , 3.42 rear axle ratio