The easy way was to plan that from the start. You can carry on as you are doing by doing it bit by bit, but its time consuming and may be prone to problems

This will teach one valuable lesson that all new modelers learn. Plan ahead and then model, if you know that the project requires individual peices in the model then model those parts seperatly.

I would have broken it down to:

Cab
Body
Door Left
Door Right
Trailer
Canopy
Chassis

The list it endless, and depends on the complexity you need. Another reason for doing each part on its own is to allow easire UVing and then texturing.

I know this isnt really that helpful, but unfortunatly you have modeled yourself into a bit of a corner.