Lack of Power

Hi, just joined today and have introduced myself.
I have a Peugeot Boxer 1.9 normally aspirated Diesel. ( Motor Home)
Parked it up last October and covered it for winter, everything was ok and working fine, uncovered it this week as the weather is getting better, to find battery flat, charged it up and she started a treat, so decided to take it up the road to blow cobwebs out, very sluggish could not get it to hold top gear and would not rev above 2000 came back home in case it packed up on me, have tried it again since and it is still the same. It will rev right round the clock when it is stationary. It was fully seviced last August and I have done about 350 miles since then.

Has anyone had a problem of this nature.
Would appreciate some help.