A better door thermometer is fine, but I found a significant difference at grill level when I got a Maverick ET732 and used the bbq probe on the grill I was cooking on. My guess is the water pan deflects heat around it and throws off the readings as compared to the shelf the food is on. Personally I'd go with the maverick and not be too concerned with the door thermometer.
POS ChinaMasterbuilt XL (demoted to cold smoking duty)
Bull Big Bahanga gas grill
Of all the things I've ever lost, I miss my mind the most!