@ TT (Texas Turtle)
Primarily, do not confuse the Android Gmail App with the Web Browser version of Gmail, they are different. The Gmail App on your Nexus 7 will allow to go to IMAP Trash and place a check in each box one by one and delete the selected emails and yes this is a PITA. You can also open the Web Browser on your tablet or your PC and go to the "Web Based" Gmail and select the trash folder and select "All Emails" and delete all of them at one time. Yes, we have another PITA. As TT pointed out she has other options on her phone, but this would be a customized version of Gmail, which was purchased from Google by your Phone supplier. You can select a different email app from Google Play and many like my choice Enhanced Email
, a paid app or many free apps that will allow you to have the option to "Select All."