Thanks to all of you for your replies and valued information. This is a bit overwhelming for me as we are venturing into unchartered territory for us, so the wisdom from those of you who have gone before us is much valued. I am looking at Ram 3500 DRW's. I am amazed at the spec differences in payloads, TC, etc. Two seemingly identical vehicles with different payloads, etc.??? I don't get it. I am grappling also with whether to go with a mega cab and 6'4" bed or the crew cab with 8' bed. I would like more room in the cab. I did test drive a 2020 Ram 3500 Laramie Longhorn today. It had a yellow sticker of 5400. It did not have the high output diesel. The salesman told me if we were going to be towing in the mountains a lot we would benefit from the HO as it comes with the Aisin transmission. 3.73 or 4.10? It seems the new 2021's have higher payload ratings than the 2020's.
Payload will vary vehicle to vehicle depending on options. More options, lower payload. However, that said, once you get into basically any modern DRW (dually) pickup truck, you have "enough" payload. In other words, stop looking at the stickers, look at the back of the truck. See 4 wheels? Then stop worrying about payload, you've got it handled for anything that Grand Design makes.
An 8' bed is nice for towing. The length makes it more stable, but also, it's nice for backing up because you can't hit the cab with the RV without getting beyond 90 degrees. It's not a must have, you can get slider hitches (that move the pivot back) or you can just be careful backing up and not get to such an extreme angle. Best case, get an 8' bed. If that's not an option, get a slider. If that's also not an option (they are expensive and heavy), then just be careful.
If you're going diesel, IMHO, 3.73 is almost certainly "enough" gear for anything made by GD. 4.10 will have a little more pop, but will cruise at higher RPMs. If you're going to spend most of your time hitched/towing, I'd prefer the 4.10. If you're going to tow a few times a year and do a lot of around town driving, I'd prefer the 3.73. Neither would be a make/break for me. 3.73 will get it done, and 4.10 will burn a little more fuel. For my use, if I was ordering today, I'd gear up to 4.10 because ~60% of my miles in the truck are with a trailer. But if the dealer said "We have everything you want over there in a truck, but it's 3.73 so we'll give you 3K off the price of ordering", I'd take that and never look back. Just not a huge deal in either direction for me.
You can't get the big cab and 8' bed in a Ram? If so, that stinks. If you're not stuck on Ram, Ford has that available; that's what I have (big cab, 8' bed, F450).