Just did this today . . .
You CAN replace the gas tank filler hose without removing the bed OR dropping the tank. At least I did this today on the 97 extended cab model I have. I think it's the 18 gal. tank. I know from the manual it DOES make a difference which model you have as to how the tank is situated. And yes I had the network of cracks all around the perimeter of the hose, just above the clamp on the tank end. And yes it DID leak if filled all the way full.
At any rate, I had read posts by people claiming they did this job without removing the bed or dropping the tank. So I guess I'm just hard headed enough to take a crack at it. Also I have the sprayed in bed liner so I figured it'd be a pain to get those bed bolts loose. You CAN get to the hose clamp from the "back side" of the tank by reaching up over the drive shaft and feeling your way. A mirror and flashlight come in handy. I managed to get the clamp loose with a stubby screwdriver.
The first tricky part is trying to get that hardened hose off after you loosen the clamp. I applied some PB Blaster around it and gave it a little time to soak. Once you remove the 3 screws around the gas cap the "filler" end of the hose is free to "swivel" around. This will help you break the seal on the hose down at the tank end. After that it's just a matter of applying enough upward force from under the truck to pop the hose loose from the tank. Work the hose back and forth until it comes off. Fortunately it's a one piece hard plastic tank, so you don't have to worry about pulling the neck out. Also the clamp is retained onto the hose by a piece of rubber so you don't have to worry about it falling into the tank.
Anyhow it took me about an hour or so to get the hose off the tank. The cheap Chinese replacement hose I bought had the filler cap end put on exactly 180 degrees off from what it should have been. So I had to spend some time loosening and pulling down the outside hose and removing the inner clamp, then turning the filler end so that the 3 screws would line up. If you find yourself in this situation, DO NOT just rotate the metal end to make the holes match up. If you do, you'll find yourself in the club with those who have to drizzle their gas in verry slowly because that inner hose will get kinked inside. My advice is take back the part or return it if you can. I had bought mine so long ago I didn't have that option. That's what procrastination will get ya.
So to get the new hose back on, that is the nut of this job. There is enough room below the bed to position the hose end over the tank neck. The problem is getting leverage to push down on the hose. My advice is to soak that end of the hose in hot water and/or use vaseline or KY jelly to lube the end of the hose to help soften it and make it slide down over the neck. (Be sure to remove any water before you install it of course. Anyhow it took me a little while working left-handed from the "front" side of the tank to finally push the new hose down over the neck. The replacement hose I had was a very tight fit. I'm not sure I even needed to tighten the clamp. But I did tighten the clamp using the same stubby screwdriver (from the back side of the tank) that I used to take off the old one.
Be careful of the wires on top of the back side of the tank and the tubes up there. I think you'd have to really try to mess up anything. And, yeah, FWIW, if I did this again I'm pretty sure I'd look into getting that bed off. But I'm here to tell you, it can be done. And I'm not having to smell those gas fumes any more either. As always, YMMV and good luck!!