I'm new to this forum but have spent many years towing different types of trailers using different tow vehicle configurations. Additionally, I thoroughly research exactly this same topic regularly. So I have an opinion that I will offer for your consideration.
To start with, trucks are just that, they're trucks. They are designed to carry and pull, if you want a capable one then you must concede the notion that it should ride as a passenger car. Some heavy duty trucks may feel smoother but at the least should be rated to carry/pull the heaviest load you anticipate. If you anticipate a 5th wheel is in your future a diesel engine 3/4 ton is a poor choice that you will regret when you do the actual number crunching as far as payload capacity. The reason for this is that the diesel engine and transmission combination are significantly heavier that a gas engine/transmission and that will reduce the payload of that truck to a number that will not, per the manufacturers rated spec, carry what you have, especially a truck with with all the bells and whistles. For example, my 2013 GMC 2500 SLE gas engine truck has a rated payload of around 3000lb. That same truck with the diesel engine/trans combo is rated around 2300lb. The 700lb difference will take the diesel out of consideration, if you want to be safe and legal. These numbers are different for every truck depending on equipment level, numbers from a manufacturers website are not accurate except for a bare bones work truck. You MUST read the sticker in the door jamb that has the actual payload available for that particular truck. With my 10,300lb Cougar 30' 5th wheel and all the gear I will carry in the bed (hitch, tools, wood, barbeque, spare propane, gas for genny, passengers, pin weight of the trailer, and all other items), I weigh in at just under my max rating of 9200lbs. I have also performed many significant performance modifications to make the truck/trailer work better under max load but I do not think that these mods increase my payload. Any tow vehicle with a door sticker payload under 3000lb will be overloaded when you load and equip it for a 5th wheel, in my opinion.
Does my gas 6.0 perform well with this heavy of a load you ask...in short, yes. But on the steepest inclines the transmission will drop into a lower gear raising the RPM's to upwards of 5000 while I try to maintain highway speed. As a former amateur road racer rev's don't scare me and the 6.0 GM powerplant is designed for this. Those same hills would not be a problem with any modern diesel engine. My mileage from new has averaged an even 12.0 and I would say that 70% of those miles are towing, heavy.
I hope that this helps
Cheers