@ol' grouch - man you know your stuff. It makes my head spin. But I understand. I think I am set on 3/4 ton and possible diesel - year 2000+?. However - since I will buy used I guess I will need to do some homework on brakes and rear end ratio. It's really actually cool stuff...just makes it tougher to shop for the right truck. But....it must be that way. News to me about the diesel oil capacity by the way. Wow. Oh it would be a class 4 hitch I need as well?
I work in logistics so I deal with it all day every day. One sort of picks up details after close to 40 years. Generally, I deal with loads up to 40 tons. There is a local trailer/ hitch/ truck cap dealer that generally has good advice for lighter towing. They sell equipment and usually have the latest in stock. For instance, the old hydraulic trailer brake controller has been replaced by solid state electronic. A couple of wires and some adjustments replaced cutting brake lines ad such.
If you have a local RV dealership, especially one that sells trailers, talk to the guys in the parts and service department. They can give you advice on how to set your rig up. When I bought my B, they gave me good pointers that made camping more enjoyable. Go with a 3/4 ton Ford or Ram (used to be called Dodge Trucks) unit. I used to like Chevy/GMC trucks but they have such poor quality control the last few years, I no longer so so. When you recall more vehicles than you sell, there's a problem somewhere.
I think you mentioned buying a used truck. There should be a label that gives the order specifications when the truck was built. It will be either in the drivers door jamb, inside the glove box or under the hood. Look for the axle or axle ratio. I like to run in the 3.7 to 3.9 range for towing. If it says something like antispin, that's usually a bonus as both wheels will pull equally. On ice though, it can make the rear slide so you have to watch the throttle in icy conditions.
On the receiver, if you're getting a bumper pull trailer, you want as heavy as you can get. Class IV at least. You have to watch the ball height. My truck sits so high, I have to have a 6 inch drop and it still makes the trailer lean back. You want the tow height to be as level as possible. An equalizing or weight distributing hitch to go into the receiver is a good idea. (Two names for the same thing.) If you think about getting a fifth wheel trailer, see if you can find a truck with a gooseneck hitch in the bed that flips down. This lets you pull the trailer but use the truck as a truck when you flip the ball down.
When you buy used, you are limited to what's available. It may take a while to find the right truck for you. Don't get in a rush. You're on the internet now, use it to look around. There may be the perfect unit just down the road and they are getting on in years and down sizing.