motorhome weight

Just had a towbar fitted to my motorhome unfortunately they had to reinforce the rear subframe so now I think I may be overweight but there are no weighbridges in my area so I carn't check.

Decided to go ahead and get the van up plated anyhow but how do I go about it. There are firms that will do the upgrade but they charge an arm and a leg and as it is supposed to just be a paper exercise it must be possible to do it your self.

Anyone up plated there Motorhome and how do you go about it.