This was found on a UK site may be of help on changing the weight limit
Several threads here deal with the question of payload on Auto Trails - and a web search offers suggestions (and services) how to increase of decrease it. I thought it would be useful to set out my free, definitive, simple six stage, tried and tested approach, which allowed me to increase my payload in the space of two weeks.
Every Fiat base vehicle leaves the Fiat factory with a metal VIN plate mounted at the front of the engine compartment. This is a Fiat 1st stage plate. It reveals the information about the base Fiat vehicle before conversion. It is of interest, for it will tell you about the maximum permitted weights for this type of chassis, a combined vehicle and towing weight, and separate figures for each axle.
The Tracker FB is built on a Fiat Ducato X250 35 'Light' chassis’ - originally homologated by Fiat with a Maximum Authorised Weight of 3650 kg (as shown on the Fiat VIN plate under the bonnet).
At conversion – Auto Trail as multi stage body builder, have ‘Type Approved’ the Tracker FB to both 3500kg and 3650kg. This permits Auto Trail to give their customers a choice - as drivers who passed their car test since 1997 are restricted to 3500 kg. Auto Trail then simply attach their secondary weight label to every vehicle that leaves their factory showing the specified gross weight – usually 3500 kg by default.
Because the model is type approved to either gross weight, it is a simple task to change – either up to 3650 kg or down to 3500 kg – without making any technical alteration to the motorhome.
Do note that, whilst the vehicle remains ‘Class 4 for MOT purposes, the difference in Maximum Authorised Weight changes the vehicle category from Private Light Goods (PLG) to Private Heavy Goods with some minor speed restrictions, or French toll charges (if they check). Revenue tax rates change slightly, but importantly, if you have passed your test after 01/01/1997 you will need to upgrade your licence. For those reaching 70 years of age, you will need to provide a completed medical report indicating your fitness to drive an HGV https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/369458/D4_160414.pdf
Six stage process
.For convenience, this specimen is for an upgrade from 3500 kg to 3650 kg:
Go to ‘contact us’ on the Auto Trail web site http://www.auto-trail.co.uk/contact-us and send a message to the Technical and Quality Manager, as follows:
“I am (your full name name) the registered keeper of the Auto Trail (model) specified below, currently with a Maximum Authorised Weight of 3500 kg. I seek a letter of confirmation, re-issued Certificate of Conformity and new secondary weight label so that I can re-register the vehicle at 3650 kg.
Vehicle: Auto Trail motorhome
Model: Tracker FB
Base vehicle: Fiat Ducato
Chassis no: xxxxxxxxxxxx
Reg. no: xx xx xxx
1st reg: (date)
Specification: factory standard
Auto Trail will send you a letter, together with the other documents requested. Photocopy the new secondary weight label, then affix it over Auto Trail’s original label on your motorhome.
Download, print and complete DVLA form 70 (if currently taxed) or form 85 if not currently taxed using these links V70https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/364271/V70_180914.pdf
V85https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/360106/V85_140714.pdf.
Complete box 7 of your vehicle’s UK Registration Certificate (log book) specifying the new authorised revenue weight and change from Private Light Goods (PLG) to Private HGV or vice versa, sign and date.