I've have this problem also with my 1995 Hymer B544 on a Fiat Ducato chassis/engine. 2.5litre TD
The airflow regulator is faulty. I was going up hill on the motorway in Portugal and lost nearly all power and ended up crawling along in 1st and 2nd which made the engine start to get hot. Pulled in and called my breakdown people who got us to a garage nearby.
The garage identified that it was the airflow regulator problem but couldn't source a replacement as Bosch (who make the part) apparently stopped producing it in Feb 2015.
It's Bosch part number 0281002087. But other dealers can also list it under their own numbers as follows:
Fiat DEALERS PART NUMBER: 77579230 OR 7757923.
IVECO DEALERS PART NUMBER: 98424639 OR 0000098424639.
RENAULT DEALERS PART NUMBER: 5010236261.
I have found one available in 2 days via Iveco but they're asking €1100 for it!! Probably because they realise that demand will now well outstrip supply since the part was discontinued by Bosch!
So I'm now trying to find a cheaper one elsewhere. In the meantime the garage here cleaned the existing part and put it back on and the van is running okay, enough to hopefully get on to the next port of call and sort the part and get it fitted.
If anyone knows a source for this part please let me know asap
And happy travelling