So far noticed this when towing at slower speeds up moderate hills and steeper.

Not noticed it without the caravan....

Quite a heavy vibration from the drivetrain somewhere...

At steady speeds (50 ish) I have noticed the revs don't seem very steady.

I suspect the torque converter, but am open to other ideas!

if it does turn out to be the TC , you may want to look up Dr trannys shudder cure , available off fleebay