Click all of the "View trimmed content" buttons in a single Gmail thread. Alternatively, just open the Print view for the thread.

clickShowButtons.js

// note that the on-screen JS has a $ but it is not true jQuery
function findButton(){
  button = $('[data-tooltip="Show trimmed content"]');
  if(typeof button != 'undefined' && button.id){
    return button;
  } else {
    return FALSE;
  }
}

// clicks a button if it finds one
function clickButton(){
  button = findButton();
  if(button) button.click();
}

// cycle through 100 times. Gmail limites messages in a single thread 
// to 100 so this should be a safe cap
for(var i=0;i<100;i++){
  clickButton();	
}

killExpandButtons.js

// note that the on-screen JS has a $ but it is not true jQuery
function findHideButton(){
  button = $('[data-tooltip="Hide expanded content"]');
  if(typeof button != 'undefined' && button.id){
    return button;
  } else {
    return FALSE;
  }
}

// if we find the expand button, remove it from the DOM. Also finds and
// removes a related whitespace element to keep things as clean as
// possible.
function killHideButton(){
  button = findHideButton();
  if(button){
    buttonClass = button.getAttribute('class');
    whiteSpaceArea = $('div[class="' + buttonClass + ' h4"]');
    if(typeof button != 'undefined' && whiteSpaceArea.id){
      whiteSpaceArea.parentElement.remove();
    }
    button.parentElement.remove();
  }
}

// cycle through 100 times. Gmail limites messages in a single thread 
// to 100 so this should be a safe cap
for(var i=0;i<100;i++){
  killHideButton();
}