Question for experienced drivers on a 13 speed fuller

I am use to driving a autoshift truck that requires no manual shifting and has no clutch, but recently i just got moved to a 13speed. I learned how to drive on a super 10 a few years back so i am kind of familiar but i am having a little trouble going into low gears.
I have always been taught to float and only use the clutch when starting from a dead stop.

every time im in town and need to go down to a crawl I try to shift from 5th direct into 4th but i preselect and push the hi/low switch while im in 5th and try to go down into 4th, but i can hear a loud clunk in the tranny,why is that. i was always told not to switch from high to low in neutral always preselct is that true?

how do you go go about shifting from high gears to low gears? do you preselect the high into low? do you have to clutch or what?

the truck has over a million miles on it, so at first i thought it might just be a bad tranny but im not sure.

How do you guys do it?

Help would be greatly appreciated.

 

When i pull out from 5th with the lever pre selected into low, as i go through neutral it makes the clunk and 4th doesn't catch, no matter how perfect the rpms are. Some times i have to come to a complete stop and start back up. I have even been on a straight away and shifted down from top gear just to see what the problem is and as soon as i come down to 5th direct, the low gears don't wanna catch. but i never use the clutch since no one taught me how to double clutch because supposedly "its wrong".

How do you guys down shift to low? Do you preselect it in 5th or wait to come through neutral?

 

yes i do shift from direct to direct but only if im coming to a rapid stop like if its a short exit.

But what do you mean by without pushing down the switch? I push down the switch in 5th right before i try to go into 4th, but there's a clunk and then 4th doesn't catch.

 

I watched alot of videos on youtube but when i do what they do the clunking is still there.

BTW i dont mind the autos when going through congested traffic, but i hate it in the snow, going over donner if you forget to put it into manual before starting up hill with chains on, the truck will gear up at low rpms and start lugging.

But i would rather trust the auto than this 13 speed, i get screwed when going from high to low.