Change a Tree Grid View's parameter values

Tree Grid View have parameters. You can set them like variables and then manually change them when you view an Item. Anyone ever changed these parameters programmatically? I'm able to reference the correct parameters and set their nodes' textContent, but the changes don't actually apply after the method completes. Here's a snippet:

var tgv_id = "some GUID"; // tree grid view id

var tgv = inn.newItem("rb_TreeGridViewDefinition", "get");
tgv.setID(tgv_id);
tgv.setAttribute("levels","2");
tgv = tgv.apply();

var startValue = bg.getProperty("_start");
var endValue = bg.getProperty("_end");

var paramItem1 = tgv.dom.selectSingleNode("//Item[keyed_name='startDate'][@type='rb_QueryDefinitionParameterMap']/user_input_default_value");
paramItem1.textContent = startValue;

var paramItem2 = tgv.dom.selectSingleNode("//Item[keyed_name='endDate'][@type='rb_QueryDefinitionParameterMap']/user_input_default_value");
paramItem2.textContent = endValue;

tgv = tgv.apply();