Sounds a bit like a dirt problem between the AC heat exchanger and the radiator.
The road dirt,dead insects,dried leafes get stuck there..
The radiator is quite easy to remove,flush it with a garden hose,from the inside to the outside.Maybe use a mild dishwashing to soak it for a while,letting all the hardened dead insects getting a bit soft.
If the radiator feels heavy somehow - it could be stuck with limestone,get some cleaner used in coffee makers.It will remove it
Do the same with the AC heat exchanger,from inside the engine room to the outside.Leave it inside the car,just flush it very good !
An other source of trouble could be your water pump.
The cheaper replacement ones have the pump wheel shrunk on the shaft inside the pump,it comes lose quite often.
If you don`t know what waterpump ist fit in your car - a new waterpump would be best,but don`t buy a no name,use febi or SKF.
And don`t worry about pulling a trailer,with a fully working coolant-system - no problem. I found an old pic from a few yrs ago...2,5 ton behind my 300TD non Turbo ! Drove about 450 kms from northern Denmark to here with it,the temperature gauge was around 100°C all the time ! (AC off !!)