I do usually get about 22 on a decent run, which is why I immediately noticed the difference. I don't think I have a heavy foot but I usually cruise about 70 on the motorway and 50-60 on dual carriageways.You seem to have been getting poor mpg for a long time, I would have it checked by someone who knows his way around diesels ..... unless you have a heavy right foot.
I had the same van as you for 5 years. I got around 22 mpg which could stretch to 24 if a lot of the journey was motorways. The 2.8 jtd is a dependable lump and if you keep between the 2,000 - 3,000 rpm range for best economy you should get what I got.
What top end speed are you doing on motorways and dual carriageways? You need to tootle along at around the same speed as HGVs. If you push it well over 60 mph your economy will drop right off.
With the amount of mpg drop off you have I think your motorhome would have caught fire if it had been a brake disc stuck on but it is worth checking them anyway as the design of the linkage is poor. It is often difficult to set up the handbrake properly as I think it was modelled on a 2nd Century Roman baggage cart.
I also go along with the Mouse theory.
One of the last jobs done before we went away was getting the handbrake adjusted; maybe I should get that checked again?
And I'm definitely getting the air filter checked