Thanks for the autopsy by the way. I have only been testing what goes in but this year the RV hasn't used as much because of the China virus so before I topped it off after the last 2 trips I thought I would test it. If it wasnt for you discovering the flap the tester tube might have been caught. The filler tube is short enough for my tester. The top of the tank where the fill tube enters was easy enough to access and so was the flap.
Thanks again @Bobnoxious.
Thanks for the additional input. Please correct me where I may be wrong. Disconnect the clamp from the filler tube and lift tube out of rubber fitting. From the picture above, inside the tank the filler tube appears to be about 1.5 inches long (any idea of diameter) and the spring small diameter so not very strong. Using a plastic device to hold the flap open, insert plastic tubing and pump fluid from DEF tank. Remove plastic tubing, reconnect filler tube and clamp, refill with fresh DEF. I'm not trying too oversimplify and know that care must be taken to achieve good results. I'm willing to try but just curious if anyone else has actually done an extraction and replacement of DEF using this method.