As long as the valve is closed and you're not occupying the Jeep, I don't think there are any laws against carrying it inside the Jeep while it's being towed. I would certainly strap it down in the Jeep (or put it in a propane carrier mounted to the Jeep, if this will be normal operation for you).
As far as carrying it on an open deck rear hitch mounted carrier behind the Jeep, while being towed behind the motorhome... I would refrain, personally. In a rear end collision, that propane tank will be the first casualty. I do realize, this is not much different than carrying a Jerry can of gasoline on a spare tire rack mount on the back of the Jeep. I'm not a big fan of those either though.
As a member of the unfortunate club of folks who have escaped their burning vehicle and watched it burn down on the side of the highway, I'm a little more fire safety concious when it comes to things that can go BOOM! than some who haven't had the experience...