38-45 can be all done in UD2. You can get around 1 level up per 20 or even less minutes.
RP2 as a place is a genuine bad experience. The place in my opinion is only useful if you have someone who can blast everything away with 1 hit, even then, if that's the case (if they are powerful enough) then they can solo UD2 and take you there for much quicker leveling.
True, but remember to kill the bosses with the low level character, so that it gives full exp