The amount of the fine depends on how far over the MTPLM figure on the weight plate your motorhome is. Less than 10% the fine is £100, 10-15% its a £200 fine and for 15% and over its £300. However, if your motorhome is more than 30% over the weight plates MTPLM figure you will receive a court summons instead. This could incur a much higher fine as well as other penalties. Details on the DVSA fines can be found through this link.