Slipping trans and no reverse?

If its slipping in high and no reverse, The high drum is pouched.
If its slipping in low and no reverse, the L/R band is in need of adjustment, or its pouched
If its slipping in all forward gears, and no reverse, then the Forward clutch is pouched, and either the L/R band or the Highdrum is pouched.
If its slipping in Drive/breakaway, and no reverse, it may be the rear roller-clutch.
So the plan is; As noted above check the fluid level in NEUTRAL,on level ground and oil hot. If the oil level is correct the pan needs to come off and the band adjusted. If its pouched you will be able to tell.The oil will stink,the pan will have debris in it, and the band anchor may be laying in the pan.But; If the pan/oil is normal,and it accepts adjustment then you can button it up and roadtest. If no change, the tranny will need an overhaul.
If the oil is stinky and dark, I would go straight to the overhaul.