I have good success with additives like Slick 50 and its newer derivatives. They can be added to the oil, the trans and the fuel system. They easily pay for themselves.
You can try a performance air cleaner that can be cleaned. There are some good iridium spark plugs available. Wires and injectors should be fine. Quality fuel cleaners can help keep injectors clean. Consider a tuner made for MHs. Monitor your exhaust manifold temp.
You might consider going to a larger diameter exhaust system.
With these kinds of add ons I have gotten 23.8 mpg out of a Ford diesel empty and around 13.5 pulling a TT.