If the OP is looking for a three year-old van then he is in luck. Look inside the gas locker and you will find a plate/label that shows the true MTPLM of the caravan - which is usually at or close to (but never over) the maximum axle load. If there is no label then (at least with Al-Ko) there will be printed label stuck o the axle casing usually on the back or bottom and towards the centre. (Advice - use a proper camera to read it, not a phone!)
Subtract the MTPLM from the maximum axle load and you can then judge whether it is worth paying £60 or so for the upgrade against the extra load benefit. Indeed if the upgrade is less than about 40Kg the manufacturer may not even offer it, bit if it is nearer 100Kg ...........
(Our U4 Seville is 1203Kg 'empty' and plated 1326Kg MTPLM, but the gas locker figure is 1450Kg so for £66 we got an extra 124Kg which is a no-brainer.)