I had this problem with trailer mounted dehumidifiers.
If the gross train weight ( total kg of van and trailer) exceeds 3500kg , you have to have a tacho fitted, this caused massive problems as the Nissan Navara pick ups we ran at the time did not have the necessary socket fitted in the gearbox.
There are exemptions though , the main ones are,,
domestic use/not for hire or reward.. lets caravanners off the hook..
Showmen, fairgrounds etc..
This is the clever bit ,,emergency water supplies , we used this as we were carrying out emergency drying, only got called on it once , the traffic cop called his sergeant who could not be bothered with the hassle, I kept records of the incident, to save problems in case we ever had an incident and we had any more tacho bother.
Having recently run trucks , tachos are hassle , and expensive to maintain and administer, avoid if possible.