The short answer is yes, it's entirely possible to have RA in your jaw - RA affects joints and your jaw is a joint. In my personal experience, I find jaw pain one of the most difficult to cope with, it's as if it permeates everything, making it impossible to ignore. As you doctor ordered an x-ray of your jaw? It might help to see if there is any RA activity there. It's good that you went to the dentist to make sure that you don't have an infected tooth, as that can also cause jaw pain.
I'm going to go out on a bit of a limb here, as what you describe sounded familiar to me. Has your doctor or dentist ever mentioned the condition called TMJ Disorder? It stands for Temporomandibular Joint Disorder and affects the jaws and it's hard to diagnose. HealthCentral has a number of pages related to the condition - you can read more here. I've had such pain on and off for years and struggled with finding a way to treat it in a meaningful way. It wasn't until I found a small page on the Internet that talked about how TMJ pain is often related to shoulder issues that I started being able to find relief. I discovered that the muscle that starts at the shoulder and moves up through your neck to your jaw was key in terms of treating the pain. I had been focusing my efforts on the jaw itself to no effect for years, but when I started treating it from the shoulder and up the neck, the pain eased. When you described your job consisting of lifting and carrying heavy items, it rang a bell. Try treating your shoulders with heat or ice (you'll find out what works best for you, sometimes, maybe both), maybe take muscle relaxants when you go to bed at night, do strengthening exercises for your shoulders (check with your doctor) and maybe it will help you the way it helped me. If it does and your shoulders are involved in this, given your job, it will be difficult to make it go away permanently, though.
A few tricks on managing jaw pain. When you're jaw hurts, try staying away from hard and crunchy food that requires a lot of force to chew, as well as "tall" food where you have to open your mouth wide to take a bite. Go with ground meat instead of steaks and pork chops, cut your food in small, easy to chew pieces and when you eat fruit, cut it in pieces, as well. Also, when things are bad for your jaw, soak a washcloth in hot water and apply it to the size of your face - TMJ pain seems to respond to damp heat. Be careful that it's not too hot, though, you don't want to burn yourself.