Strange 3.7 ton , and I asume 3700 kg MPVW/GAWR/MTA because driverslicence B in Europe is for up to 3500 kg.
But OK then we go from that .
What you have to search for is the VIN plate wich has Maximum permissable axle loads on it , mostly I = front axle and II is rear axle.
Then from tires the maximum load or loadindex and the pressure needed for that or loadkind.
Probably European tyres C-tyre ( C for Comercial, USA LT( lighttruck)).
Mostly used are D-load/ 8PR ( plyrating) with pressure needed for maximum load of 65psi/4.5 bar or 69/70/4.75/4.8.
Then when you are searching also searc on sidewall for the speedcode.
This is mostly Q speedrated for up to 160km/99m/h.
Give those data and I will put it in my made Motorhome tyre/tire-pressure calculator, and give a picture of it in answering post.
Greatings from Holland from a self declared tyre-pressure-specialist.
The strange lighting problem can also be because of bad earthing,-connection ( if I use the right word) . Search for a wire for that from Chassis to the lights .