Volvo 2006 Vn670 Isx 500

Hi guys I Have Volvo vn 670 2006 with cummins ISX (500 hp) engine.
I was going 6.3 mpg last year was runnig the best from my past trucks and I was going 70mph most of the times.
The power was great, no matter how heavy you are its like you dont feel the thing.
All that was great till one day I heared a big Boom nexst to my egr cooler and turbo. the thing that happend is there was 2 screws broken on engine manifold. I drive it like that 130 miles to nearest cummins service shop.
They had to drill the old screws out and put the new ones in.
I also replaced the EGR cooler which they toled me it was leaking coolant.
On my out of service they toled me that turbo is near to death also, so is EGR Valve. I didn't have that much money to replace all of them at same time, so I went to earn some money for repairs.
During my driving i noticed the truck had no power like it did before, and it waists alot of fuel. I checked from fueling to refueling i was going 4.5mpg to 5.9mpg max. I told them that there is a problem with my fuel efficiency and they replaced EGR valve 2 times, new Turbo, New turbo accelerator valve, new boost pressure sensors and few more sensors on the way, ac compressor, alternator, belts, starter.
I found out someone took my fuses in side fuse box, so i replace them also. Most of the things I changed on my truck, now i got an electrical fault such as, check ligh control module, brake ecu, abs light on, on my dash board when my lights are on the battery lamp is flashing every 5-15 min. when i go over bumps on road my marker lights on truck shut off or on and the power goes bad and good. idk what to do anymore the power has not changed,and so fuel milage. Anybody can help me?

 

Thank You so much I'll see what i can do, and report it here.
I have one more question that how much volts is supposed to be for electronic sensors when they are active?