Was it the actual tree or the tree points at the end of the gullet plates?
Use a girth without elastic for a better fit
Re a harness having a crupper:
The crupper on a harness is not holding a saddle of the type you ride a horse in, girthed up tightly with the weight of a rider in it, from sliding forward. In the case of the OP, the crupper would be pulled forward with the (riding) saddle, so would put quite a bit of pressure on the underside of the tail.
I agree that a crupper is not a very good solution.
Problems like this are why “saddle area” is evaluated as one of the points during inspection for breed registries. Some horses have issues because of their shape, where the “girth groove” is, etc. which make saddle fitting difficult. But, a good saddle fitter and a well-fitting saddle should help the situation–probably not completely fix it if the horse has a poor saddle area but it will be your best bet.
All these pads, foregirths, etc. are trying to compensate for poorly fitting saddles and/or poor saddle areas. And there’s only so much that can be done.
I had a similar problem, however a thin line girth solved it for me.
A proper driving horse doesn’t have a constantly shifting weight on his back, pulling on the crupper, either. 
