Since using RMA’s in Magento 2 is an Enterprise-only feature, there is not much information out there on what classes to use.

Below is how to programmatically load an RMA by ID in Magento 2:

<?php 
    /* 
     * RMA Helper 
     * -- 
     * Created by Joey Babcock July 2020 
     * Last modified by Joey Babcock July 2020 
    */ 
    namespace Joeybab3\RMA\Helper;
    class RMA extends \Magento\Framework\App\Helper\AbstractHelper 
    { 
        protected $_rmaRepository; 
        public function __construct( \Magento\Rma\Api\RmaRepositoryInterface $rmaRepository ) 
        { 
            $this->_rmaRepository = $rmaRepository;
        }
	
        public function getRmaById($rmaid)
        {
            $rma = $this->_rmaRepository->get($rmaid);
		
            return $rma;
        }
    }
?>

As you can see it’s not really that complicated but due to the lack of information I figured it might be nice to have more resources on it.