Well you have quite a few responses but here is my $.02.
Cummins 5.9 (ISB) engines get between 9-11 mpg avg., some have seen as high as around 12...but I think that is coasting down hills with a tailwind.
I have a big block C8.3 Cummins in a 36 (36'-10"...
) and I average 8-8.5 mpg, with my Jeep GC toad (+5100#), 7-7.5 mpg. A big block M11 Cummins will barely break 8 mpg on average but has a LOT of torque.
The smaller Cummins engines can be uprated to around 350HP and 800 Ft/# of torque but they are not really designed for this. Later models are built to withstand the extra power and torque though.
Caterpillar engines vary as well. Stay clear of the older V-8 Cats as they are more expensive to repair with higher mileage. The newer C7, C9 etc. are very good engines but most Cats dont avg. better than 9 AFAIK.
The Journey and corresponding Itasca (same difference) are very good coaches. I think they primarily came/come with Cats but it depends as some I have seen have Cummins power.
There aren't too many 35 footer DPs as far as I can recall so look carefully. I would check on the various RV web sales sites as well as some of the better known dealers and see what is around. You might not find a better time to buy than now.
Oh, and a big diesel engine isn't even really broken in till it hits over 150K miles. My C8.3 is rated as a "million mile engine" with major service not due until 500K...allthough it will take me a long time to get there and I am a bit AR about preventative maintenance anyway so...