Much to share. Not all of it good. Almost all of it educational.
Will start with your question(s) and follow with play-by-play.
The filler and vent hose ... They were deteriorating but still functional. Covered with black powder that I guessed was the outermost rubber failing. Important note on my rig was that there are two places where there are short runs of rubber line/hose. First is where the fuel tank filler neck and vent hose take a hard turn from the inside of the RV wall to the metal line that turns and then connects to the fuel tank with two more short runs of rubber line. Hose source? Measure the internal diameter of the hose and either order off Amazon, or better still take it into your local autoparts store and put it on them to get you the correct size.
Removing the tank: Took great energy and a fair amount of ingenuity. WARNING. There is virtually no slack on the three fuel lines that connect to the fuel pump. If you don't have long slender arms with great grip strength, find someone that does. You need to reach up over the frame, on the port side of the tank, where there's about 4" of clearance, and blindly detach the fuel lines. Same for the electrical connection to the fuel pump, which mounts on top of that frame rail, and the Fuel Pressure Sensor which also runs up over that rail. There are also two vacuum hoses that attach to two separate ports, but they had a bit more slack. I cut new vacuum hoses a wee bit longer and that helped a little in reassembly.
Dropping the Tank: It's heavy. I would guess 200lbs. Maybe 250. Completely drained it. Six bolts hold it in place. Don't be distracted by the additional three bolts on the starboard (passenger) side that clamp the tank together. Do not touch them. They do not need to be loosened and will not loosen. I used three jacks to get the job done. One 12 ton bottle jack to raise the port dual wheels so I could get under there and work, and two racing jacks positioned at the front and back of the tank to lower it. Plus lots of jack stands to keep me from getting crushed. Once the tank was down I used a block of wood and a rock as a lever and fulcrum to get the racing jacks out, then dragged it out from under the RV. I had dreamed of doing it all with the tank just resting under the RV, but that was impossible
Once the tank got out into the sunshine, things got ugly. As mentioned previously all six bolts sheered off trying to get the old fuel pump off. I ground flat, drilled and tapped the remaining bolt stubs which were welded in place on the inside. The only apparent torque needed on these six bolts is to compress the rubber gasket that seals the pump in place (I hope). I was fortunate to find a new Delphi fuel pump that was a visual match to the old pump. I also replaced the fuel pressure sensor with an identical unit.
Replacing the tank: There are two welded-in-place eyelets on top of the tank in opposing corners. I took two cargo strap ratchets and hooked them forward and aft of the tank, hooked them to the tank, and routed them up over the frame. Was able to raise it up enough to place the racing jacks back under the tank. It took about 12 attempts to position the tank so it would exactly match up to the holes (port side) and bolts (starboard side). Reattaching the three fuel lines and two electrical connections is again challenging in that you need to do it blind by reaching up over the frame once the tank is in place.
NOW FOR THE HEARTBREAK! Got it all together. Turned the key. Started with a little pump on the gas pedal. Ran rough. Could keep going if you pumped the gas. No pumping the pedal meant it stalled. About the third start I was pumping away, thinking I was clearing a fuel injector issue, when I heard BEEP BEEP BEEP, and everything went dark. Zero power to anything. I gave up and went in for the night.
More soon.