Questions:
Regarding the decision to change the gear ratios in axles, this is mostly done when a significantly larger than OEM tire is installed.
> Does tire size (height, diameter, profile) determine which gear ratio should be used? Is there a chart of optimum ratios for "X" tire size, for "X" weight/type vehicle?
Tire diameter is the dimension that determines the gear ratio. I'm not aware of a vehicle specific chart but the information is pretty standard. If you zero in on a specific vehicle and look at builds, you will see lift and tires and maybe gear ratios. If not, you could ask. A gear vendor would also be a good source.
If you put larger tires on, it is like always starting out in second gear and you may never use fifth gear. With an automatic transmission you might not notice it as much but its not optimal for many reasons.
> Is this what happens if you put bigger tires on and don't change gear ratios?
Yes. You can increase tire diameter to a point without needing to change gear ratios but after that is will be sluggish and hard on the drive train.
When installing selectable lockers, the gears are removed so if you know you want larger tires, it best to plan ahead and do it all at the same time.
> Are the removed gears replaced with new gears in a new ratio?
If the original gears are in good shape they can be reused.
Usually paired with a suspension lift.
> What dictates the amount of suspension lift, the tire diameter or the new gear ration?
Usually a person identifies a tire size they think is necessary to navigate a given terrain or carry a certain load. They then determine if the vehicle needs to be raised up to avoid interference between the tire and vehicle body. (Most lifts and big tires running down the highway are purely to stroke the drivers ego LOL).
For a camper setup I would stick with a slightly larger than stock tire that performs well and doesn't require lift or gear change. Adding a camper raises the center of gravity enough without lifting the truck.
Too large a tire wont fit under the bed and will need to be mounted on the outside adding another required mod.
Distilling all of this down, you will likely want a 1ton truck with single rear wheels. If the truck doesn't have a factory limited slip rear end, a selectable rear locker would be awesome. Keep the stock gear ratios and run a slightly larger tire. Trim fenders where necessary. If you find you are limited by tire size you can always increase those things later. The smallest lightest camper will go the furthest away from maintained roads.
The camper question is still a conundrum. The way I see it is you can go cheap and get a couple seasons out of it or drop some coin on a custom build and keep the design basic and light weight.
