This is actually quite a grey area in terms of MoT testing. Technically, the MoT tester is correct insofar as vans with a gross weight (GVW) of 3000kg to 3500kg require a Class 7 MoT, but there is an exemption that exists allowing professionally converted motorhomes up to 3500kg GVW to be tested as Class 4 - if they are registered as motor caravans on the V5C registration document. This is at the discretion of the tester, though, as not all Class 4 MoT bays are large enough to accommodate such a vehicle. In reality the difference between Class 4 and Class 7 tests is negligible in terms of pricing and the items tested are broadly the same so you could get it tested at a Class 7 centre without too much hassle.