The short answer
Measure your rig before you plan a trip: overall length, overall height including anything on the roof, body width, and the height of the lowest point with the tanks full. Then plan the route against the tightest restriction on it, which in Europe is far more often a bridge or a town gate than it is a campground.
Height is the number that stops you
A typical motorhome is 2.6 to 3.2 metres tall, and a fifth-wheel can approach four. European bridges, underpasses and old town gates are frequently signed at 2.6, 2.8 or 3.0 metres, and campsite height barriers are often lower still. A satellite dish, a roof box, an air conditioning unit and a roof rack are all above the roofline and are the usual cause of damage. Photograph your rig's height, including the tallest fitting, and check every route's lowest point against it.
Length, width and turning
Overall length determines which campgrounds, ferry routes and parking bays you can use. Body width matters on alpine roads and in old European streets. The combination of a long vehicle and a towed trailer gives a worst-case turning circle well beyond what the tables for the vehicle alone suggest, and the trailer's wheels swing wide at corners. In practice: choose routes with generous lane widths and avoid town-centre parking in a rig above about seven metres.
Gradient, altitude and fuel
Engine power falls with altitude, so a naturally aspirated diesel loses roughly ten percent for each thousand metres of elevation, with turbocharged engines losing less but still losing. On a sustained climb, a heavy rig may need power it does not have, and a towed trailer makes the whole thing worse. Fuel planning is simple arithmetic with a hard rule: never plan to run below half a tank on a route where the next station is more than a hundred kilometres away or where you are on an isolated road.
The practical plan
Book campsites with dimensions, not just positions, and prefer sites that publish a height limit. Check fuel in towns rather than on the road. Assume mountain weather closes a pass and build a complete alternate into the plan rather than a fragment. And take a photo of each height restriction you pass, so that if you do hit one later you know the exact number you are working to.