I have checked the information I am about to give you in the manufacturers' price lists available as of October 2021 , however consider them in any case indicative because there are so many models and mechanics setups and they have changed so much over time, that it is impossible for me to be more precise. I remind you that any vehicle with a total weight on the ground greater than 3500 kg requires a C license for driving.
Fiat Ducato . There is no doubt: it is the mechanical basis of the vast majority of motorhomes. The Ducato was born as a light commercial vehicle and its structure was designed for weights ranging from 2800 to 4250 kg. It always has front wheel drive and always single rear wheels. There is also a three-axle version, used by some manufacturers for particularly large vehicles. There are 4x4 versions of the Fiat Ducato made by Dangel, a French company specializing in four-wheel drive vehicles that can be fitted to Ducato or Citroen mechanicals (they have the same chassis and transmission). Mechanical base used by all manufacturers, in any price range.
Iveco Daily.It is a light commercial vehicle built on a sturdy chassis which has the typical structure of a heavy work vehicle. It has rear-wheel drive and can have single or twin wheel axles. The acronym shown above the front mudguard gives information on the fully loaded mass (GVW), the type of vehicle and the power. For example 35C17 means GVW 35 quintals, C stands for light truck (therefore rear axle with twin wheels), 17 is the power rounded up (170 horsepower). Instead of the C, there may be an L (light van, single wheels, with lower GVW) or an S (larger van, single wheels, with GVW up to 35 quintals). There are 4x4 versions of the Daily. Mechanical base used by many manufacturers, especially in the construction of vehicles in the medium-high or high price range.
Ford Transit. Light commercial vehicle, with GVW between 2940 and 5000 kg, can have front, rear or all-wheel drive (the Trail version, used, for example, in the outfitting of some pure vans/campers). Generally used in the construction of motor homes in the rear-wheel drive version with twin wheels. Mechanical base used by some manufacturers, perhaps it was more present in the past.
Mercedes-Benz Sprinter . Light commercial vehicles with GVW between 3000 and 5000 kg, can be front-, rear- or all-wheel drive. Mechanical base used by various manufacturers in high or medium-high price range vehicles, generally in the rear-wheel drive version.
Other mechanics. As I said above, the world of mechanics used for campers is vast, you can find vehicles on Renault Masters, Citroen Jumpers (especially vans), Volkswagens (think of all Westfalia-equipped Transporters), in short, a nice assortment.