I have a 1993 HR Navigator with the mechanical 8.3 cubic inch 300 Hp Cummins and the 3060 Allison. The combination is bullet proof if it is properly maintained. We've only had one problem in the 12 years we've owned the coach, and that's when the computer control (which is under the touch pad) went bad about three years ago. The replacement was about $1,000. I've replaced all the transmission relays (in the black 6X6 box) with new relays just as a preventive measure. The relays were the same as all the others in my coach and I bought 24 in a box for about a $1 each on e-bay several years ago, so I had extras. I replace the transmission fluid about every 50K miles.
The Monaco you described has the 400 ISL, a slightly larger engine but just as reliable as the ISC, the newer version of my mechanical version. The Allison 3000 series (probably you'll have the 3060 which has an overdrive 6th gear) is the transmission of choice on almost all motorhomes and many, many medium duty trucks.
You'll love your Monaco. Since you do much of your own maintenance, it should last you as long as you'll want.
