Your CAT C7 is 350HP but I believe it's 'only' 860 lbs of torque so it might be worth driving the 380/1050 to see how it compares.
Without having driven it, I agree that the 42A might be a bit underpowered but I think the 380/1050 is probably okay for the 40G with a GVWR of 32k lbs. The Tiffin Phaeton 40QKH has the same engine at 35k GVWR.
From the specs, a good option might be a Gulf Stream Tourmaster TF40U which says it's a 425HP/1200 Cummins (ISL ?!?) BUT their website is 5 years old (brochure that comes up is a 2008) so they might not even make it anymore ...
Please don't turn this into yet another 'HP vs torque' or 'how much HP is enough' but Monaco Knight / Holiday Rambler Ambassador comes has a 40PBT with a Maxxforce 10 engine. 'Only' 350HP but it does have 1150 lbs of torque so that might work for you.
Most manufacturers do make a DP bunkhouse so just pick a manufacturer that you like and see what they offer. However, typically bunkhouses are only offered it the 'lower-end' models which usually means smaller engines (e.g. there are a few DP bunkhouses that will only tow 5k lbs).
Another option might be to speak to manufacturers about modifying an existing floorplan. Suitable ones might be a Revolution 42T (if you look at their FB page, they have pics of one they built with rear bunks) or an Eagle 45T or Newmar Ventana 4346 or DutchStar/MountainAire 4347. Another option could be the Tiffin Bus 45LP but I've heard that Tiffin won't make significant changes like that as custom orders.
As far as the fresh water capacity, make sure you are comparing apples to apples; some manufacturers include the 10G HWT in the fresh water capacity (i.e. I think Fleetwoods are 90G+10G), some probably don't.
As far as the Charleston 430BH, I've read that it was discontinued after last year (on their website it's a 2012 brochure).