Can the webmonitor be considered as a REST API? If so you can use the REST API Monitor to monitor the rest api and check the response using XPATH Expression. Navigate to Web tab > REST API > Click on the + icon > Add the REST API Endpoint url > in the content checks section, choose Response format as XML > In the XPpath Expression give your expression.
You can use our XPath validator tool to check the validity of the expression that is passed.
The second option is to have a plugin to monitor the custom value that you want. You need to write a simple custom script (either Linux or Windows flavored) that parses the XML and sends the value(s) you are looking for, then deploy it in a machine with a server agent installed (in order to run the plugin script you need a server agent to run which will send Site24x7 the custom metrics). The plugin will start to monitor the value(s) and you can set thresholds for them.
PS: Every server being monitored comes with a plugin that can be monitored, free of charge.
Let us know if this helps