Best 18Tonne curtainsider for mpg

i drive 18t daf 22 plate , mpg is around 12 ish ,however i use pto a lot (builders merchant). cab is tiny ,no leg room at 5"11
12 speed auto box is diabolical , pulling out of a junction is a nightmare it selects a lower gear than needed then when you are halfway out the gearbox has a meeting to decide which gear is next , having made the decision and selected said gear it then realises that all momentum has been lost and the whole process starts again . going up a hill and its worse ,put it in manual mode to save the embarrassment of it stalling . reversing slowly downhill automatically puts you in kangaroo mode . if however you only plan on driving somewhere reasonably flat or on motorways its ok. the 12 plate 8 speed manual was a lovely wee thing that you could actually “drive”