Hi
I am guessing this is a Rimor motorhome of the "panel van conversion" style built on a Fiat Base. You will never completely silence a vehicle of this type, though owners get adept at stuffing tea towels into the grill and cupboards etc. Window blinds can also be a bit of a noise source, sometimes having them half open is the least noisy.
The rear door strikers have some adjustment in them. They should need a determined slam to shut, and if you look at the mechanism inside at the top, the "pegs" for want of a better description must be fully upwards when closed. If the doors shut easily it suggests they are not compressing their rubber seals enough. If you don't want DIY a Fiat dealer will be able to adjust.
Finally, owners are sometimes advised to run at very high tyre pressures such as 80 PSI (5.4 Bar), a case of the tyre manufacturers playing safe. However, they are not the ones who have to put up with the extra road noise and vibration. My own MH is probably close to the 3500 kg limit, but I find no problem running 60 PSI (4.1 Bar) all round.