Anyone know if Barth/GM used the RV torque converters in the TH400 trannys they supplied with the big block P30 Barths?

The GM chassis that Barth used was a driveaway forward control chassis. I doubt Barth ever changed components on the chassis, with the exception of the tag axles they added to longer models.

The TH400 was built in two variants. The standard units had helical gears. The heavy duty ones (sometimes called the TH475) had straight cut spur gears. They were identical in outer appearance. Some of the heavy duty ones can be identifed by the parking brake drum mounted on the tailpiece.

To my knowlege they all had the same torque convertors.

I've been reading about RV torque converters that lower the stall speed, give a little more oomph off the line, help in slipping and improve fuel economy. Anyone every done this to their TH400?

When I replaced the engine on the Regal it went from 240 hp to about 400, also torque around 500lbs/ft.

Stock TH400 is only rated about 500. Since I had the original trans I decided to replace it with something more robust. Competition TH400s are made for over 1000 hp.

I wanted use the 480LE 4 speed with overdrive. Unfortunately they have electronic shifting and require a PCM (Power Control Module) which also interfaces with the modern engines. Unfortunately, my fuel injected 454 had an ECM (Engine Control Module) only. To use the 480 I would have to start over with a PCM and program chips, etc.

Decided to go with a rebuilt TH400 trans. I bought a TCI unit made for RV use, about 800lb/ft rated. A very good unit. Smooth tight shifting, shift rpm allowed engine to pull right to max and not lug down into higher gears.

Getting to the point:

I replaced the torque convertor with the recommended TCI unit. Heavier rating and claims better "lockup" at speed, ie about 4% more efficient.

That seems about correct. At 60mph cruise I was seeing 3000 rpm. With the new torque convertor it was about 100 rpm less.

The tighter shifts were the only thing I noticed when driving
I suspect the gas mileage was up to 4% better but with the numerous changes made I could not prove it.

By the time I sold it, the 33ft tag axle 454HO was getting 8 mpg at 60 mph on the highway. It was getting 6.5 when I bought it.