David,
If you are concerned about fuel mileage, you probably will not like the C 13 engine in the coach that you referred to, however if you are like most coach owners and realize that fuel cost is one of our lower expense items, following depreciation and taxes in some states, it is a nice coach. The C 13 in the 2008 Thunder will probably average around 6 MPG, and can go as low as 5 MPG depending on speed and terain.
The durability on the engine is excellent, with the exception of some problems with the DPF (diesel particulate filter) system on some engines, and if the coach is a 2008, it probably has a DPF system. The performance of the coach is good, and the Jake brake will be a considerable improvement over your exhaust brake.
Gerald