Going over 3,500kg GVW, even if you have a C1 Licence, may not be the best solutiion if short of pay-load.
Vehicles over 3,500kg GVW suffer from higher toll rates and/or access restrictions to low emmission zones in many countries.
in Ireland such vehicles are also tested on the Heavy Goods Vehicle lane at VTN test centers while those not over the magic number are tested on the Light Goods Vehicle test lane.
Current information from National Driver Licence Service states the following in relation to a Category B Licence:
Vehicles (other than
motorcycles, mopeds, work vehicles or land tractors) having a MAM1 not exceeding 3,500 kg, designed and constructed for the carriage of no more than eight passengers in addition to the driver.
The vehicle may tow a trailer (a) where the MAM of the trailer is not greater than 750 kg or (b) where the MAM of the trailer exceeds 750kg the combined
MAM of the towing vehicle and the trailer is not greater than 3,500 kg.
Reading the above in conjunction with (a) it is indicated that a motorhome not exceeding 3,500kg GVW may tow a trailer with a MAM not greater than 750kg.
However it is indicated
in (b) that if the trailer exceeds 750kg the combined weight can not be breater than 3,500kg.
See the following link for full information http://www.ndls.ie/en/Driving-Licence/driving-licence-categories.html
It would therefore appear to be perfectly legal to tow one of these http://www.spaceextender.com/Medien/Videos/Fahrten_Zusammenschnitt.html or
one of these https://www.youtube.com/watch?v=WJgUZyXQh74 to increase overall carrying capacity and at the same time avoid the drawbacks associated with vehicles over
the 3,500kg GVW mark.
Such units perform the function of a trailer, but without the instability sometimes asspciated with one. Also, the challenges encountered when reversing with a tradition type of trailer attached do not exist.