You can come close in price with either combo, especially if you buy used. A decent 5th wheel used can be had for around "20,000. A truck with a diesel may run around 30-35 used with low mileage or 30,000 new for a Ram with the 6.4 gasser. Used class C's and A's generally have low mileage on them compared to trucks of the same age, unless of course you buy from a rental company.
Bare in mind that you will pay insurance for two vehicles with the C's and A's. A trailer is cheaper to insure. If the trailer is paid off, liability is covered by the tow vehicle without any additional cost. Diesel pushers can be expensive to buy. Lots of luck.