Right off the bat, the bed is too short for YOU. I'm only 6' and my feet will dangle unless I wake up and make sure the top of my head is against the headboard. I can hear the wooden slats creak in the Murphy and I'm about 228lb - solid muscle
Now if you believed that last part, I'm selling some land......
Seriously, the Murphy is short but at 6'4" you will look pretty long and hard to find a rig with at least an 80" bed. Our last DP had one. Loved that bed. The Murphy is for shorter, lighter folks who are immune to hard slats poking their ribs at night. I admit after a week or so on the road I get tired enough to sleep on it though.
The dinette makes a shorter bed. The overhead might collapse under 300lbs but someone probably knows the weight rating. It is longer than the Murphy and on a cool night I sleep up there if the mattress is in the rig. Most of the time I pull the overhead mattress out and leave it home.
Thought - find a 24D and crawl up on the bed and stay there a while. Five minutes won't give you the wooden slat rib bruise. Tell the salesman/woman you must have a good bed.