Yup I'd also advise checking the cheapest bits first, I had the same issue with the clutch on my 200tdi 90, turns out the fluid was leaking back through the master cylinder, the rubber washers had perished, it got gradually worse over time, fitted new master and slave cylinder, problem solved, I guess also one quick check to see if the clutch is slipping is to get the truck into top gear, cruise along at say 50mph then just floor the gas, see if the clutch slips or not, not very scientific I know
but if it's slips badly then that would indicate to me that you're clutch is kinda goosed.