When I first looked at this problem and was working on a solution, I quickly gave up on using the existing tank fittings because of the poor access. John's fix using the upper fitting helps to get around this problem. Dave had already come up with a copper pipe formed into a Z, sort of, shape to fit through the existing holes and then lay on the bottom of the tank. That seems like a good idea too, but hard to install in that lower fitting, in fact, next to impossible to install.
The use of 1/2" PEX (5/8" OD) means it's too big to fit through the hole and John had to enlarge the hole to get it through. Another method would be to use 3/8" PEX (1/2" OD) or 1/2" copper tubing. And then adapt back to the 1/2" PEX outside the tank. John's curved copper example is good, but the bend is too sharp to make the corner. A hand formed 1/2" copper dip tube 90 might work very well if someone wanted to use copper instead of PEX.
I debated between using copper or PEX for my dip tube coming in through a fitting on top and decided that copper was best for me. I was able to make the holes in the pickup so close to the bottom of the tank that mine will draw down to 6% before getting air and will struggle on down to zero percent with some bubbles.
Those plastic PEX fittings that seal only with an O ring against the outside of the tube seem barely good enough and may leak with the sloshing water and vibration while driving. The drilled out brass fittings John used are much better. And they can be easily taken apart if needed.
I want to re-stress that the tank fittings, especially the ones from the factory, are not very strong and can crack if the fitting is screwed in too far. Be careful!
When using the copper crimp rings to connect the valves or splice the tubing, the tool can be cumbersome and hard to get in place. Lowes sells another one called a "pinch ring". The tool for these is more compact and works well. The rings are also stainless and can be removed easily if needed.