Like you said vehicles for the most part are outside of Zell, so it doesn’t really matter.
Organizationally I would think they would have their own tractors, no point in “wasting” a combat vehicle in moving it around when it could be used on the front lines, especially in a post-jihad setting, where we see more and more clan vehicles. I would say two tractors and their trailers constitutes a point and the tractors stay somewhat near-by for easy repositioning.
I don’t see many, if any, clans giving a line tank like the Enyo or Axel IIC a trailer to pull behind them. Pulling them behind hovers or even wheeled chassis doesn’t make sense due to the many terrain restrictions and the cutdown in speed on them will effect them more than it might a tracked chassis; taking more motive hits now even more often.
I’d wager they are reduced to the support trailers along the lines of artillery. You don’t want that in the front lines, and rather than using a combat vehicle for defense and repositioning, which have increasingly seen front line duty or at least combat for the clans, you can use a tractor and some solahma to defend it.
If for whatever reason they did put a trailer on a line unit I’d imagine they’d call it as a part of the vehicle, making the points 2x2 but only acting separately when they are dropped off.