HELP

So I gave my bike Pulsar NS160 BS6 to bajaj authorised service center for servicing. Major service after 43,000 kms. Valve clearance settings, chain sprocket replace and steering cone set replace. After the service my bike's performance is completely dead and mileage has dropped from 42kmpl to 25 - 28kmpl. Tried speaking to the service manager and staff but they refuse to accept any issues in the bike. I want to escalate this issue further but I dont know how. I have sent a long mail to customer care and also spoken on the phone and complaint has been registered. Any other help / guidance would be highly appreciated as I have spent almost 15,000 rs (5.5k for parts and 8.5k as labour charge) on this service and my bikes performance is completely dead. Literally feels a normal 100cc commuter bike. So yeah, any help would be appreciated.