why cant i downshift my 18 speed without grinding

i just bought a 89 kw w900 with a 18speed trans. im just learning everything and have not had anyone to teach me how to drive i have drove several 9 speed trucks before in the past and didnt have no problem shifting but ive met my match with the 18 speed im having problems downshifting bc i can not find the right rpm range. everyone keeps telling me to keep the rpms up. when i keep them up and try to downshift and raise the rpms even higher it will not even try to go into gear if i bog the motor at about 900 to 1000 rpms sometimes it will go in just fine but most of time it will scrape but still go in. the people that are telling me to keep the rpms up have been driving for years but my truck will NOT downshift at higher rpms can someone help me? i have a learners permit and have to get the shifting down pat before i can take the road test i am also runing bobtail in this truck right now, the 9 speeds that i drove i had empty trailers behind them, does it have something to do with the weight differance? can someone tell me what i am doing wrong or give me some tips on shifting. thanks for any help.

 

can you give me a idea wear my rpm's need to be to make a good down shift?

 

i have the upshifting down pat with or without the splitter and can even shift without the clutch and its just the second time i have drove it but downshifting is a different story i think if i could do it one time i would get it from their but i also have another question when you use the splitter and go from direct to overdrive can you just pull the spitter back and push the clutch in and go back into direct or do you have to pull the spitter back and also downshift with the shifter in order to get back in the direct gears thanks to everyone for the help.