Where I volunteer fixing cars for people in-need, when the a/c drain tube is clogged, we use a fast & effective method that requires no disturbing of the vehicle. We use a shop vac and suck the debris through.
#1 - Locate the tube hanging down under the vehicle
#2 - Place the shop vac hose there and close the gap with your hands
#3 - Listen for a change in the shop vac sound. Repeat as necessary.
#4 - If the debris refused to budge, we use a screw driver to break the blockage free, then repeat step #2. But on an E-series, the tube might be real long. If so, use a coat hanger.
Of coarse it would be ideal to clean the drip pan inside, but this gets it done fast. On many of the vehicles we work on, they are old and sometimes filled with personal belongings & dirt (some people we help, live in their vehicles) that we find this to be the better way to handle it.