The sprinter was built by MB in 2011, it was purchased, unused by the Motor Convertor in 2012

Any 'leisure batteries' and associated electrics were new in 2012

You apparently do not have the German registration date for the finished Motor Home.

The Registered in 2014 would apply to the UK registration date, If MB accepted that the first registered date was 2014 you have nothing to be concerned about. The MB warrenty is with regard the original running gear only, warranty for conversion (leisure side) lies with the dealer and possibly the origonal convertor.

Your vehicle requires an annual service which should include at least an oil/filter change, even if you only do 3k miles per year

The motorhome was registered in Germany in April 2014 and, when the U.K. dealer who imported it in November had obtained type approval, he registered it with the DVLA, which accepted that it had been registered initially in April. Presumably he supplied some kind of proof of that as I can't see the DVLA just accepting his word? The V5 now states that it was first registered in April 2014.

Furthermore, Mercedes confirmed that it was only registered in April and that the warranty started then.

I've no problem with accepting that it has been sitting arround in a showroom and hasn't been used for two years, but I do wonder if there isn't a gap in consumer legislation here. Is it reasonable for instance, that a car can sit around for five years before being registered as a new vehicle? Surely dealers should be made to inform clients of the true age of the vehicle?

It's normal for motorhome chassis cabs to be built long before the body is fitted, up to 2 years in some cases - the bigger motorhome converters tend to buy their base vehicles in annual batches, to their own specification, but can't accurately predict how well the finished motorhomes will sell.

It's not a new situation so you'd be best enquiring on a motorhome forum how the vehicle warranty cover is dealt with in these cases.

As a complete aside - did you know that the Fiat Ducato and it's Citroen/Peugeot cousins often only use the front part of the chassis cab, fitting an Alko lowline chassis from the cab backwards - to avoid wasting the rear half of the normal chassis, Fiat build a cab/engine at each end for shipment which are then cut in half to make two by the motorhome converter.