i would see a cardiiologist...actually and electrophysiologist ( a cardio that specializes in heart rythyms).
I agre with Al. It's a good idea to see a cardiologist or EP and have your heart evaluated. But I do tend to agree with your doc that a resting heart rate of about 50 shouldn't by itself make you very tired (my resting HR is normally in the low 50's and I feel fine) . The cardiologist would be a better judge of that, of course. Meanwhile I wouldn't give up on your GP. It may well be that as he says, something else is causing your tiredness. You should keep looking until it's resolved.
My husband Stuart suffers similar symptoms to you. He expereinced low heart rate and tiredness his cardiologist eventually decided to implant a pacer that helped a great deal but also had ablation for atrail flutter as the tiredness came back. His diagnosis is sick sinus syndrome. My suggestion is to keep asking questions and keep demanding answers.