The crucial number here is the towing capacity of the Transit. This model ranged form 2000kg to 2750kg (depending on body and wheelbase) so a Daihatsu car shouldn't take you anywhere near that limit. That, of course, is if you flat-tow the Daihatsu behind the motor-home. If, however, you tow the car on a trailer, then you might (depending on the trailer's size and construction) find you're getting close to that limit in some circumstances. The other problems will be the same ones faced by anybody who tows a decent sized load; wear and tear on the mechanical components and the sheer cost of fuel, particularly diesel.