I guess you’ve done the sums already but it’s worth confirming them;
look at the manufacturers weight plate fitted to the Nissan. Is the Train weight over 3.5 tonnes? If it is you now need to add the Gross weight of the Nissan to the Gross weight on the trailer plate. If this exceeds 3.5 tonnes then the use of the vehicle will come under EU drivers hours rules (when the trailer is attached) and a Tacho will need to be fitted and used. Remember this is the weight given on the plate, not the actual weight. Should also add that it doesn’t matter if the trailer / vehicle is loaded or not, the rules apply if the weight thresholds are exceeded.
If the vehicle was first registered on or after 01/05/2006 then a digital Tacho must be fitted.
Many vehicles, particularly 4x4’s couldn’t be fitted with the type of sender unit required by a digital tacho. However, there is now a piece of kit called an ‘M1/N1 adapter.’ This allows the sender to take it’s signal from another system like the abs pulse sensor and means that digital tachos are easier to fit to many more vehicles.
On days where you use the vehicle with the trailer (for anything other than the non commercial carriage of goods) you will need to obey the EU drivers hours rules and keep records using the Tacho.
On days where you have the trailer attached for part of the day you will need to obey the EU rules regarding continuous driving when the trailer is attached. You will also have to have an EU daily rest period. You will need to keep a record of all that days’ duties. Probably best to use the Tacho and use the ‘out of scope’ function for the time you drive without the trailer. If you are driving the Nissan without the trailer and you’re using it for work, for example collecting bits for fitting to cars then you will fall under the control of the UK Domestic Hours Code.
If you use the vehicle on a day entirely for private purposes eg. going shopping, taking the dog for walkies in the park, taking your caravan on holiday (you get the picture) then the vehicle use falls outside the EU or Domestic rules and no records need be kept.