Vicki -
The 3126cat is rated at 300 hp for this application or make sure it is (just a computer program change if not, so don't worry) The limiting factor for the 3126 in this application is the radiator or it could be programmed for 330 hp / 860 ftlbs torque.
Since your not pulling a race trailer or hanging out in the Rockies, you should be fine. You have to remember the 3126 is in a majority of the low to mid range pusher motorhomes. I have a buddy with a 36' pusher with the 3126 and the Allison 6 speed and has no complaints with the power. Beleive me I asked him when I started to look at the FL106. The best possible situation would be to get behind the wheel and take a test run, you may want to check with Lonnie and see where the couple of M2's he's built went and see if they will allow a test drive.
I don't think your going to find or at least very easily a full automatic used truck or for that matter conversion. Most of the late model conversion units are semi-auto (clutch to start and stop with auto shift) or manual transmission. Meritor does have a clutchless trannie, but again there are very few out there so far and they are going to be on the bigger trucks (mo money)
Now as far as options are we talking for the conversion or the FL106?
Bill
2003 28' Show Hauler Motorhome on a 1995 FLD 120
www.showhauler.com