Cleaning A/C coils: DIY or hire a pro? (See March 8 update)

That's where you need to be a little bit careful. The gunk will drip down into the condensate pan but if the drain holes are plugged up it can overflow into the trailer. Check those drain holes first. You can do that just by putting some water into the tray and see if it drains out. Generally, you don't need to rinse off the cleaner from the fins so just spray on the foam and let it do its thing. Then run the unit for a few hours to let the condensate rinse the fins off.