If yours is like mine ...
The genset fuel pump on the microlites is going to be inside the genset case to the left rear, behind the air intake. You may need to pull the genset out a bit to disconnect the fuel line (and the battery line).
To get the genset out of the B190, you'll need to disconnect the electrical from the inside under the bed. There is a box where the flexible conduit attaches and the nut at the end of the conduit needs to be released.
The case is screwed to the bottom of the compartment via flanges on either side. One screw may have a ground wire.
You will need to get underneath to undo the clamp holding the exhaust pipe to the bottom of the genset. That should be in a 4x4 hole that is also the exit for the cooling air.
Do look for where the fuel line ties in. It is supposed to go directly to an alternate feed on the front tank but mine was simply tapped into the line from the rear tank (not good as that means it pulls fuel through the fuel pump). The manual indicates that the rear tank on the Ford cutaway vans usually has an auxiliary fuel feed on the rear tank that should be used for this.
See also Onan Microlite 2.8 KV-B teardown