How do I remove the old gravity fill from the tank?

Is the hose off of it? If so its probably just a very tight fit with a little mastic behind it. Take a small pry bar on a scrap piece of plywood to distribute the force and proctect the metal. try to work it out going all the way around it like that

There's no hose attached, just the pipe connecting the pot metal fill on the outside to the tank.  If that's the part you're referring to, then no, it's still attached.  I can't actually see a pipe at all because the tank is fitted so snug next to the interior wall.  There's a bunch of goop there covering up the pipe.  



 I think this may have to be my next step.  Question for you, how do I heat it?  I don't have a heat gun, but is that what I should use?  I was thinking of getting one, so maybe that should be my new purchase.  
 

Ok here are a bunch of photos. Hopefully this gives you a better idea.

I am not trying to salvage either at this point. I just ordered a new fresh water tank and water inlet. This one is in terrible shape so I don't care if I mess it up further. I don't have a sawzall but everyone keeps mentioning it on here. Sounds like maybe I need that too! haha My tool collection is going to be pretty big by the time this project is complete.

Jannica, if it's not too late (ie, you bought a sawzall at harbor freight next to the dinette tables), you might try PB Blaster;

www.ryderfleetproducts.com/pb-blaster-16-pb/pb-penetrating-catalyst-rust-penetrant-p-blf-16pb

available at most auto parts stores.
Your filler spout is threaded onto the pipe just inside the wall of the trailer. If your tank is loose enough, push it toward the outside of the trailer and give it a good spray. You should also be able to reach it from the inside of the wall. Let it sit overnight, wrap the spout with a rag and use your channel locks:

www.google.com/search?q=channel+locks&rlz=1T4GGLL_enUS316US316&source=lnms&tbm=isch&sa=X&ei=6WrlUsS4KKjisASYs4HABA&ved=0CAoQ_AUoAg&biw=1366&bih=590#facrc=_&imgdii=_&imgrc=lp-m_MTV3FIp5M%253A%3BYshlCYxRMrK4IM%3Bhttp%253A%252F%252Fecx.images-amazon.com%252Fimages%252FI%252F717nvMoRQRL._SL1500_.jpg%3Bhttp%253A%252F%252Fwww.amazon.com%252FChannellock-440-12-Inch-Tongue-Groove%252Fdp%252FB00004SBCU%3B1500%3B502

and turn them anti clockwise. If they don't turn try gently tapping the pliers with a hammer, or spray it again and let it sit again.
It will loosen up.
I know you don't plan on using the system again, but there are others in the group who would like that filler spout to keep their rigs original, so it has value.
My '69 had the same problem and the PB worked great.
It smells, like a 30 year old lube bay at the old repair garage, so if you try it put some rags or old towels down around the inside wall to keep it from soaking into the wall wood.

I'm pretty big on conserving old parts even if I have no need for them, so this is just my take on stuff.
Good luck.

Kirk

It's too late!  Oh well, I couldn't conserve the tank anyway, it was ruined as you can see from my pics.  The metal pot fill could still be salvaged if I remove the piece of pipe.  I already ordered a new one though because I wanted a plastic one.  That way I don't have to worry about this type of thing again.  Thanks for the advice though!

You are welcome to the part, I'm going to switch to the plastic one.  I know it's not vintage, but I am tired of dealing with the metal fittings.  Just let me know how to get it to you.  You can send me a private message with your address if you want, and I'll put it in the mail.