Will the solar panels pay for themselves? Doubtful. If they did, everybody would have them. Should you get them anyway? I did and I'm glad.
We have 1050 watts of flat-mounted solar panels, much more than most people. On an average good day, they generally produce about 300 amp hours. 300 amp hours x 12 volts = 3600 watt hours, which is 3.6 kilowatt hours. Assuming electricity is 10 cents per kilowatt hour, that's 36 cents a day of electricity, or about $10/month worth.
The amount of electricity you use to charge your batteries while your motorhome sits at home is way less than that, so your savings will be much less, too.
Depending on where you camp, the solar panels will likely mean less generator usage. At the very least, you can use the generator to do the bulk charging and the solar panels are much more efficient than the generator at filling up that last little bit of the batteries.
I'm not familiar with your Monaco Knight, but if it's a fully loaded DP like my Alfa, it uses a surprising amount of electricity doing nothing. We calculated that one of our 175-watt panels is used just to handle the parasitic loads (e.g. appliances that are turned off but plugged in use electricity) when we're boondocking. It helps a lot to unplug things like TVs and microwaves when you're not using them.
Our on-board generator is loud (not contractor generator loud, but not quiet Honda generator loud, that's for sure) and there are places we boondock where people are around and I just won't inflict that noise on them. Tragically, not everyone feels the same way so I get to listen to THEIR generators, but I'm counting on karma coming through for me at some point, I hope before I'm dead.
When we were wrestling with the decision, Mr. dbnck finally boiled it down to, "If you think it's the right thing to do, then it's the right thing to do." I decided it was. I'm a green type, so the idea appealed to me from the start, but it was a lot of money and solar panels don't exactly form themselves cleanly from sea water or anything--they have an environmental cost.
Now a few years later, I'm really really happy with the solar. We'll go for months without ever needing them in a big way, but when we want to boondock somewhere for a few weeks, they sit up there quietly toiling away.
BTW, we started with 700 watts of panels. We're fulltimers so we have computers and we use the microwave and other creature comforts on the inverter while boondocking. We did fine on the 700 watts, but we didn't always completely recharge the batteries every day, which of course isn't a big deal at all except to somebody like me. Plus, solar panels were going up in price, so we went ahead and got two more.
The 1050 watts is way more than we need when we're in South Texas in full sun. When we were in Colorado in October/November, however, with a lower sun, shorter days, and some passing clouds, we needed every bit of it.
If you do go solar, map out your roof as if you were going to blanket it with panels, and place whatever panels you do get in the designated spaces. We had to design around the shadows cast by our motosat dish and vent covers. When we put the two extra panels on there, we didn't have to move the others, although I have to pick my way around them fairly carefully.
Here's a picture
of our roof. The two panels at the bottom are the ones we added.
You can also get tiltable mounts for greater efficiency, but when we were buying, it was the same price to add an extra panel as it was to get the tiltable mounts. However, I'm a neatnik and it would be nice to be able to tilt the panels if for no other reason than to clean under them. Then again, our roof is over 13 feet off the ground, and we like to go where it's windy, so the less time I spend up there messing with tilting mounts, the better.
And get a charge controller that is bigger than what you think you'll need, so you won't have to replace it if you do end up getting more.
I guess solar panels are like children or pets--they're not going to pay for themselves, but some people get them anyway because of other attributes they offer.