1. Mechanic/Repair Availability
The more mass-produced vehicle will probably have a larger network of possible repair places.
Also, smaller size allows more repair shops to handle.
2. Parts Availability
Ditto
3. Costs of Repairs and Parts
Ditto
4. Safety tested, airbags, etc.
Vans will likely have an edge over Class A motorhomes. Airbags, electronic stability, crosswinds, etc.
Exception: Class A motorhomes built on a touring coach or bus (e.g, Prevost) will have an integrated body and chassis, and may have many safety features.
Years ago (haven't checked recently), RV.org favored Class C motorhomes over Class A motorhomes (non-Prevost type) as the Class Cs used a full-size van front. Many Class A motorhomes back then had inadequate front protection.
5. Handling
Many Class As had too long bodies (same was true of some 15 passenger vans).
Some Class As on Prevost type platforms can perform better than vans.
Vans are closer is size to a large SUV, and thus easier to drive.
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Obviously, Class A motorhomes have their own advantages.