What are you using as treats?
You need a high value reward, what's worked well for me: hot dogs, pieces of steak, freeze dried liver, Whiskas Temptations cat treats, Zukes and that should be enough, lol.
Remember, YOU CAN'T BE ANYWHERE NEAR THE TUB!
Most of Buddy's fears, unfortunately are "you" related. And it's not your fault. We've all dragged a dog into the tub before. But this is why I must stress, and why you being in the tub did not work.
The combination of you + tub= Buddy is afraid.
That's why I want to take you out of the equation, and let buddy and the tub meet alone.
This isn't an "exercise" that should be going on constantly. You take buddy into the bathroom and close the door. Work in the bathroom, for about 10-15 minutes and then it's over. Back to normal life.
Remember I said, don't expect him to jump into the tub the first few times. If he even nears the tub, to get a treat that's great.
Leave a trail of treats towards the tub. Maybe 10 pieces of hot dog steak whatever. And GO AWAY! Go stand in the corner and do NOT make eye contact with buddy. As soon as you do, he will assume that your going to throw him in. Pretend he isn't even there. Ignore him, look at your nails, fuss with your shirt, find something to focus on far away from Buddy.
This is what usually happens:
The dog will watch you, make sure that you aren't going to throw him in. Maybe be stressed and nervous, go to the bathroom door and cry to be let out. (You still haven't spoken to him or made eye contact) After a few minutes, and a dozen times of making sure that you aren't going to throw him in, they will sniff the food. Look back, make sure that you aren't approaching, and eat a treat. After the first few, if it's a good high value reward, they should start snarfing down the treats that aren't too close to the tub. The ones that are on the bathtub's edge, or the two that are within a foot or two, they'll look back at you again. STILL do NOT make eye contact! And hopefully, usually, then the dog will snatch them up and back away!
For the first time that is great! It's exactly what we want!
I must stress though and not to be mean: The more you talk to him, look at him and are near him, the less chance there is of this working.
Buddy is afraid that YOU are going to throw him in the tub. So the closer you are to him and the tub the more his fear will grow.
YOU have to:
-not make eye contact
-be as far away from Buddy and the tub as you can be
-seem uninterested in what Buddy and the tub are doing
So here's my new advice, well a mix of old and new: lol
-get a more high value treat
-make the session short and sweet, 10 minutes two or three times a day
-make it a session, so it does begin and end, your not constantly waiting for him to go near the tub (leaving treats overnight)
AND LEAVE BUDDY AND THE TUB ALONE 
LOL
Pretend that they aren't even there.
I'm glad you haven't given up. From Buddy's end, you have to understand his fear.
I'm sure bathtime was a frightening experience, and depending on how often it happened, it may take a few weeks to get him comfortable.
But, I do know, that if you follow those instructions to the T, within a month or two, you should have a dog, that may not enjoy the tub, but will comply, as long as you are calm and confident when it comes to bathtime.
Good luck!
