I am sure that access varies from one model to the next. For what it's worth, access to our furnace on our 139 is limited but not impossible. There is a metal panel across the front of the furnace, inside the trailer. It unscrews.
Once I have the panel off, I use a long crevice tool attached to my shop-vac to clean the insides of the furnace.
You may also be able to get to the furnace from the outside -- there is a metal grille that unscrews. I have never taken off the exterior panel because I don't want to have to re-caulk the seams. But I am sure it can be done by someone with more ambition than I have.
If you are feeling really ambitious, you could make a long extension for your shop vac out of a piece of garden hose, using a piece of PVC pipe as a reduction bushing. Duct tape is essential, too. I have made this type of an extension for other projects but have never used it on the furnace.