Its hard to say, there is that and as you've written, running the larger tyres as well, will put a lot of pressure on the drive train. Remember sticking your hand out the window as a kid. There was quit a lot of wind resistance to push against at the 100kph. You are also having to accelerate that little bit harder with the weight of the roof rack, and steel bar bull bar, if you have one.
Did I also mention my camper is based on a small trailer?
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