Congrats on your new set of wheels... We're now on year 3 with our gently used 2004 Allure High Cascade model. It too has the ISL370, and we've found it to do what we need it to do. We're getting about 7.5-8.0 MPG, keeping the speeds down to around 62 when on the road.
A few things we went thru, to see if they apply to your coach too:
1) Fuel hoses to generator and Hydro Hot, did not handle the ULSD fuels very well, and can form little pin leaks. We ran new Gates Barricade fuel lines to these.
2) The OEM shocks, Koni Adjustable SP1 (First generation of the adjustables, now up to SP3 generation which are a bit more robust.) were undersized by CC for the front end IFS duty. Our unit, also the 40' with Tag, two slides so a bit less weight. Would have problems with the front end bouncing more then I liked, and would bottom out too easily when hitting dips. Tried setting 5, the highest, and helped only a little bit.
So, we swapped out all of 6 shocks with Road Kings. If I had it to do over, I would probably have done drive and tag with Koni Adjustable SP3 on setting 3. But I still would have the Road Kings on the front end.
3) I also added the Super Steer Motion Control Units (MCU's) to the front and rear air lines. These are 'check valves' that help the coaches air suspension 'set' a bit faster when shifting loads from side to side, like in S curves. Hendersons Lineup suggested I go with the under 30K Lbs for the front IFS, and over 30K Lbs for the rears - I did this. And noticed a better handling coach right away from these being added.
4) If not yet done so, change to full Synthetic and OAT based cooling fluids is a good thing to consider. (Read up on the Cummins section on these, Spike45 provides good input and advice.) I went ahead and flushed and changed the Onan QD8000 over to Final Charge too, to keep all of the cooling fluids the same.
I do suggest you get the four corner weights, and set your tire PSI per your manufacturers charts.
And hey, first go use this rig for a bit, get the feel for it. Read over the maintenance receipts on the rig, as much of this may have been done already. And you may find the handling is just fine they way it is for you, and that is fine
Enjoy, the three slide Newport is a very nice coach, we walked in a few of them. I'm not an expert by any means, but drop me a PM if I can ever help on anything specific about 04 Allure era coaches. (Traveling to Alaska for a few months, so maybe slow to respond, based upon Internet coverage...)
And of course, this board, and the Yahoo CC Owners User Group, are a wealth of knowledge!!!
04 Allure 40', Tag, ISL370, 2 Drivers Sides, Michelins XZE*, Road Kings, SS MCU's, Xs 4 L16 Lifeline AGM coach batteries, 1200W Solar Panels with Midnight Classic 150 Controller, Bus-Kote, 3 Headed Side Mirrors added, Grey to Black added water pump (increases boondocking to about 85-90 gal of Grey), Centramitics on F/D/T wheels, Crossfires on drive duals, Optilube and Amsoil Cetane in each tank, etc., etc.