[SalesForce] Salesforce CPQ Custom Template Content (Quote Templates) not showing field value from custom controller

I just created a simple quote template. With 1 Template Section. I have assigned that section with a Custom Template content that references a visualforce page I created.

Visualforce page,

<apex:page contentType="text/xml" controller="CustomQuoteController" showHeader="false">
    <apex:outputText value="{!Quote.SBQQ__BillingName__c}"></apex:outputText>
</apex:page>

CustomQuoteController,

public class CustomQuoteController{

       private final SBQQ__Quote__c currentQuote;
       private SBQQ__QuoteLine__c[] currentQuoteLineList;

       public CustomQuoteController(){
            SBQQ__Quote__c[] currentQuoteList = [SELECT Id, SBQQ__BillingName__c 
            FROM SBQQ__Quote__c WHERE Id = :ApexPages.currentPage().getParameters().get('id')];

            if(currentQuoteList.size() > 0){
                 currentQuote = currentQuoteList[0]; 
            }
       }

       public SBQQ__Quote__c  getQuote(){
             return currentQuote;
       }


       public SBQQ__QuoteLine__c[] getQuoteLines(){

               if(currentQuote != null){
                     currentQuoteLineList = [SELECT id, SBQQ__ProductCode__c 
                     FROM SBQQ__QuoteLine__c WHERE SBQQ__Quote__r.id = :currentQuote.id];
               }

               return currentQuoteLineList;
       }
}

SBQQ__BillingName__c name doesn't display when preview quote template document. It's just blank. Anyone know why ? What am I missing ? Do I have to change something ?

When I remove contentType="text/xml" and preview the visualforce page it shows the SBQQ__BillingName__c value.

This all also in Salesforce CPQ.

How to make it work and show up in quote document ?