In 2011 we had a similar situation- bought a 5th wheel and searched for a good used truck capable of towing this particular 5th wheel after we had researched all the numbers
For GVW, GCVW etc. We found a 2004 Dodge dually 3500 long box that was 6 speed standard, and was right up our alley. At that time it had 125,000 klm (Canadian) about 78,000 miles and it now has over 400,000 klm - around 260,000 miles. It’s been a good truck for us and with age a few things are needing replacing occasionally (alternator, water pump, clutch, air conditioning etc) have all been done) but we don’t usually spend more than $2000/ year average on repairs such as that. Interestingly, that truck would probably sell now for about the same as we paid for it back then.

I advise if it's a fifth wheel to get with way more payload capacity than you would think. Safer, and you'd be surprised at how fast weight adds up as you add things to carry. Judging by what I see towing large trailers, many people are likely overweight on rear axles and tires, which can cause parts to wear faster and be scary if emergency braking or maneuvers are needed.
People seem to do OK with short boxes these days but will have to pay more attention in sharp jack-knife truing, and I'd think with the shorter wheelbase, sway may be more of an issue in strong winds than with an 8-foot box crew cab.

Thanks, LYRV Peeps.
Nice to know we’re not being over-cautious, then.

What about suspension. An RV repair guy told us that coils are the thing now on trucks as the market demands a smooth ride over a work truck. Husband wants a model with leaves instead. Ray, you have a newer truck…..what do you have for suspension on that newer truck of yours?

I'm not sure about today's truck, but when I bought mine, the coil springs were only an option for the Ram 2500, all the Ram 3500 came with leaf springs. I guess for heavy-duty applications, leaf springs were still the best. My previous vehicles were 1994 Ford F350 and 1989 Ford camper van and they both had leaf springs so I've never had rear coil springs to compare.
When the Ram 3500k was brand new, the suspension was quite stiff but has worked in over the years. Now I don't notice it with regular driving, also carry about 500 lbs worth of stuff in truck boxes, so that helps. It's very smooth hooked up to the trailer weight.
I only notice now when I go off-road without the trailer on really uneven ground, it can get a little jerky to drive, but heavy diesel trucks aren't that great off-roading anyway.