The boost pressure gauge was fairly simple on my DD 8V92TA. There was a plug in the air horn between the turbo and the intake. I installed an 1/8" tubing fitting where the plug was and ran the 1/8" tubing on the outside of the bus into the drivers window, temporarily. There I used a 30 PSI gauge, you may need a higher pressure, from the local hydraulic hose store, to check the boost pressure. I had 22 PSI at 2100 RPM in third gear on a steep hill. You will have to select the gear that gives you enough load, yet allow the engine to get up to governed speed, on a steep hill. To be sure there was no restriction in the air filter I installed a new element for the test. Probably not necessary if your air filter doesn't have too many miles on it. Then compare the PSI reading you get to the Specs for your engine, which you should find on the internet. Checking the fuel pump pressure would probably require removing the pump and taking it to a pump and injector shop. They should be able to tell you if the pump pressure is within your engine's parameters. For exhaust restrictions look for partly crushed pipes, especially the tail pipe after the muffler.
Did your bus come from the factory with the L10 engine? If not, maybe the transmission shift points aren't right for your engine. You can manually shift down to keep the engine speed higher, to keep the engine from bogging down too. Still hope you can find the solution without too much expense.
Good luck, Sam