LT35 engine problems

[Siosal]

Hello folks.

I've got a VW LT35 2.3 Mercedes petrol engine with an LPG conversion. I bought the vehicle 300 miles from home and it cruised on the motorway nicely. Problem was, the van would stall instead of idle.
I took it to my local commercial dealership to get fixed. £900 later and a new throttle body had been fitted. Great, no more stalling.
Gradually from then on the VW has lacked power up hill and even on some flats, with fuel consumption ridiculously high. This was diagnosed as a faulty exhaust valve's flap jamming. £300.
Less noise from the engine now but no greater performance. Faulty MAF was next suggested, only to find that the 2.3 Mercedes petrol engine doesn't have one. Yet another diagnostic test and an oxygen sensor fault was throwing up faults with 2 out of the 4 cylinders, so £200 was spent and a new sensor was fitted.
Just as bad as ever. I've now been advised that the turnover valve may be causing the problem. Please could somebody help me out here because it's obvious that nobody from the dealership has clear knowledge of my engine as they all describe it as rare. I'm paying out sums of money with no end result. I can't help but think that the new throttle body has caused this, but the dealership disagrees and seems to want to work it's way around my entire engine.
Thanks in advance for any help

Siosal