Things I can think of:
- check air pressure for all tires on both the moho and toad so that they are equal.
- make sure the toad's front end is aligned properly so it does not pull to one side.
- make sure the steering wheel on the toad is not locked.
- make sure the towbar raise between the toad and moho is within allowable tolerance (it varies for the different models of towbars - even from the same manufacturer).
- especially if you are using a drop hitch, remove any play in the hitch receiver with an Immobilizer (blue ox) or similar device.
- if an adjustable towbar, make sure both sides are locked.
- make sure you don't have a dragging brake or bad bearing on the toad that might pull it to one side.
Do you notice any difference between driving in the left lane vs right lane (freeway of course)? It might just be that your rig is sensitive to the crown in the road.