How you load your van is
crucial to its stability.
"Gross Trailer Mass, or GTM, is the mass
transmitted to the ground by the tyres of the
trailer when coupled to a towing vehicle and
carrying the maximum load recommended by
the manufacturer or importer, approximately
uniformly distributed over the load bearing area."
In short, it is the limit of weight that can be placed
over the axles of the trailer. It does not include the
weight exerted on the towball.
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gorv.com.au
"Aggregate Trailer Mass, or ATM, is the total
mass of the trailer when carrying the maximum
load recommended by the manufacturer. This
includes mass imposed onto the towing vehicle
when the combination vehicle is resting on a
horizontal supporting plane." If you were to put
your caravan on a weighbridge on its own and
resting on the jockey wheel, this is the weight you
would measure against ATM. Basically, it is the
maximum total weight the trailer can be.