Help with tow vehicle choice

I'm new to the RV and tow vehicle world, and after doing a not insignificant amount of research I have realized that the enormous amount of information out there can be a little overwhelming. I was hoping to get some direct opinions based on personal experience from anyone nice enough to help out so I can have some actual real world advice, so any help would be much appreciated!

Right now, I am planning on purchasing a travel trailer for my family later this year that will likely be in the 30 ft range and 7-10k lb range. But I don't have a tow vehicle yet, and I want to make sure I buy enough truck so that in a few years if I want to move up to a 40 ft 5th wheel that weighs 14-20k lbs, I am not limited in my choices.

First off, I have only been to 1 RV show & had time to do a little internet and lot shopping, so do those length and weights seem appropriate for the larger RV's within each of the classes (travel trailers and 5th wheels)?

Now, on to the tow vehicle.

I have a family of 6 so was wanting to get as much room as possible in the cab of the truck for comfort (obviously with a bench in the front). For this reason the Ram mega cabs are intriguing to me, but should I be worried about the fact that they don't have the option of the longer 8ft bed? Along those lines, how reliable & safe are the sliding type of setups for 5th wheels that people use in the shorter beds?

I live in the south where there isn't really any snow, but hope to travel to most of the national parks at different times during the year. I don't really plan on being off road on our trips, and will be using the truck for minimal things outside of family trips. Given that should I spring for a 4x4 or will I be safe with a 4x2?

Will I need a dually to cover the weight of a potential larger 5th wheel? Do the dually trucks tow better across the board?

In looking at the Rams, there is also the potential of getting the Aisin transmission which would increase the reported two capacity from about 18k to 28k lbs. Is this what people would recommend (or the similar step up in other truck makes).

I could obviously just go with everything upgraded (4x4, Aisin, dually, etc), but all these things are expensive and I don't want to stretch my budget any more than I have to if they aren't necessary.

Also, in general anything else that you can think of for me to consider getting in the tow vehicle would be appreciated (I figured I would get a tow package with extended mirrors, an integrated braking system, 5th wheel setup, etc).

Thanks in advance for any help or guidance y'all can give me and I look forward to potentially seeing some of you on the road!