Some food for thought. 1) The ISL, or CAT C9, will many times get you 2 Stage Jake Compression. vs Exhaust;
2) The ISL vs ISC gets you the mentioned 1200 LBS Torque, and depending upon specific coach and year, should be between 400-450HP; 3) Tag vs Non Tag searches are worth reading. Yes, they do add weight to the coach (Which should also get you the ISL or C9.). The key thing Tag's do for a 40' DP is more CCC, higher stability while driving, especially in windy conditions, and the Drive Axle is closer to the Steers - so much more maneuverable when moving around and into a space in say a campground. (Our 40' Tag, turns tighter then our 28' Bounder without IFS. Sure, you have to be aware of tail swing... But much easier to maneuver and park in tight areas.).
Another benefit of going to a Tag, is usually (NO, not ALWAYS) it gets you higher up in the food chain of a manufacturers coaches. This usually yields better chassis, and better built houses.
Last comment is on the years you're considering, 2008-2012, understand the shifts in smog levels on the engines. An earlier smog level engine, sometimes is an easier engine to maintain thru the years.
My general advice to someone starting the journey for a DP, is to:
Determine a budget range, set aside $10-15K for Post Purchase expenses. (Catch-up on any deferred maintenance items, to establish a solid baseline for you to maintain from that point forward. As well as minor interior mod's to make it 'your coach'.) Go out and kick many tires, sit in many coaches (Slides out and in) and visualize livability. Narrow down to a list of Manufactures and Models and years that you like, then start the search. In general, I always recommend dropping years to remain within budget, and buy the highest quality model coach that meets your needs and that you like. Quality lasts. And the higher quality models, usually had superior chassis and engine/trans combinations - and these make a good foundation to carry the house forward over the years.
Last list of suggestions, is continue to research some of these features, to determine which are, or are not, important to you:
-Side vs Rear Radiator
-Tag vs Non Tag
-Drums all around vs at least Disc on Steers
-CCC
-Aqua/Hydro/Oasis heating vs Propane Water and Furnace heating
-Heated tile floors vs none
-Raised Rail chassis construction
-Slide out generator vs non slide out
-Xs 2 AC vs X's 3 AC
-Jake vs Exhaust
-3 Way Fridge vs Residential Fridge
-Tank capacity (Grey/Black tanks sizes - can often dictate how long you can boon dock - if important to you.)
-Slides qty. (Each slide adds additional weight, and weakens the house structure. Each slide can take about 6" on each of the slide (1') of wall cabinet space. Each slide can also lower upper cabinet space by 6-8". Each slide adds complexity.)
-Leveling Air vs Jacks vs Both
-Front Entry door vs Mid Entry Door (Majority of DP's are Front, but some Mid exist.)
Only you two can decide what is, or is not, important to the two of you... But I'll share that I'd rather have an older higher quality DP with larger block engine (C12, C13, DD Series 60, ISM), Tag, Rear Radiator coach - then a newer entry to mid level/tier quality coach.
Opinions vary and the most important thing is for the two of you to do what you feel is right for you...
Happy hunting, and best to you and all,
Smitty
