This question has been answered by multiple community members. See the first response.
$doc_id = $modx->getOption('doc_id',$scriptProperties,$modx->resource->get('id'));
$tv_id = $modx->getOption('tv_id',$scriptProperties,0);
$output = '';
if (!empty($tv_id) && $tv = $modx->getObject('modTemplateVarResource',array('contentid'=>$doc_id,'tmplvarid'=>$tv_id))){
$output = $tv->get('value');
}
return $output;[[getRawTVValue? &tv_id=`25`]]