If you're planning on launching the bass boat with the motorhome, I would suggest getting a front hitch installed for that purpose. The long rear overhang would make for interesting boat launching in many cases (and the imperfect visibility from a motorhome doesn't help any either).
I would prefer an E450 or equivalent, even if not towing the boat.
Edit: The Coach House Platinum 220 is smaller than you're suggesting and has no slide outs, but has two twin beds that can be made into a single king bed in the back. Their Platinum 271XL is about the size you're thinking of and has a single slide out and the two twin beds at the back, but the web site doesn't indicate that they can be made into a king bed; however, it might be possible, and I'd at least give them a call to find out before discounting it entirely. Coach House units are quite good quality...and, of course, priced accordingly.