Covered grill? Hibachi? Can you setup indirect heat? Many questions so, a pictures or nomenclature would be useful for people chiming in. I mostly go low heat and smoke cooking so, I won't be much help. I can provide what I consider to be the finest recipe for baked potato.
Fry up few strips of bacon, roll potato in bacon grease, pierce a couple of times with a fork, roll potato in sea salt (large pieces), do not wrap in foil then you are simply steaming the potato. Indirect cooking is best, over coals means close monitoring. Cook until the potato is soft when gently squeezed. The skin will be crispy, the salt, when crunched, will fill the mouth with flavor and the creamy soft inside will nearly melt in the mouth. Top with all your favorites, butter, shallots, the fresh bacon bits from earlier, sour cream, etc.
I run the temp up while the meat is sitting and redistributing the juices. At 300 the tater should take 30-40 minutes. A potato nail will shorten the time. The key is the potato feels soft inside when done.
For your tenderloin: Make sure your meat has plenty of fat which will melt into the meat and keep it moist. Injecting will help as will marinating over night. You don't have to eat the fat but, it does moisten meat. I do full pork shoulders for sandwiches. I insist the butcher leave the entire fat cap on. Make sure your meat sits for, I like a half an hour, so the juices flow back into roast. Meat continues to cook when removed from the heat so try not to over cook.