Hi, Linda
I used to have one of these and it really was (in my opinion) a very, very good van.
Unfortunately i sold the handbook with the van so i may be of limited use, but you should be able to reverse up a slope with no problem unless the slope is so steep that it is bringing the over-run brake into effect (and that isn`t something you can readily quantify in a place like this), or the brakes are incorrectly adjusted/damper shot.
If you or your friends/family (trying really hard to P.C here, not at all like me!) are not mechanically competent it may be worth getting an engineer to check it out, as if the latter is the case it will affect your braking on the road, and thats not worth the risk.
sorry I can`t be more conclusive.