It is not in a manual "mode" it's locking out the last over drive and showing you the gears that it's using.
From a different forum and from many years ago;
Here is the full operation and the differences of ERS and Tow/Haul modes explained for the 09-11 Ram 545RFE transmission. 2012 has the same transmission renamed to a 65RFE and has a slightly different operation.
Let’s start off with the gear ratios of the 545RFE. There are 6 possible gear ratios but the truck will only let you use 5 of them at a given time.
1st 3.00:1
2nd 1.67:1
2nd Prime 1.50:1
3rd 1.00:1
4th 0.75:1
5th 0.67:1
In normal Drive mode when up shifting you will use gears 1, 2, 3, 4, & 5. In ERS or Tow/Haul mode you will use gears 1, 2, 2P, 3, & 4.
So, when in normal Drive mode:
1=1st – 3.00:1
2=2nd – 1.67:1
3=3rd – 1.00:1
4=4th – 0.75:1
5=5th – 0.67:1
When in ERS or Tow/Haul mode:
1=1st – 3.00:1
2=2nd – 1.67:1
3=2nd Prime – 1.50:1
4=3rd – 1.00:1
5=4th – 0.75:1
As you can see, ERS and Tow/Haul lock out 5th gear (0.67 ratio) but adds use of 2nd prime in the up shift pattern. ERS and Tow/Haul are the same as far as shifting order and gear limiting goes, Tow/Haul just holds each gear longer and downshifts much sooner when slowing down for engine braking. ERS and Tow/Haul mode also disables the use of MDS.