I've spent most of my time over on the toy hauler forum but since we've been talking about moving from the TH to a diesel pusher this is where I'll ask my question. We've been looking at 34 to 36 feet...
Make sure you look what engine, you already mentioned ISL, and could probably get away with ISC....do not go for the ISB (660 ft/lbs), just not enough torque. For the Cat engines stay with C9, the C7 at 860ft/lbs is really not enough.
I used to have a 39' with the C7, towing a 20ft enclosed with a Rhino and 3 dirt bikes and was always wishing for more power.