We always stay off site, so I can hopefully give some insight on that.
First, just a thought on showing up that early for RD. If you show up just 10 minutes early and wind up waiting 30-40 minutes for 7DMT, that's 40-50 minutes spent waiting (and more time sleeping). If you show up 90 minutes early for RD, that's less sleep and 90 minutes spent standing in a cattle herd. I understand the flip side of it, in that if you're that early for RD, you can be done 7DMT by 8:20 if it's an 8:00am opening. But personally, I'd much rather get the sleep and not stand in that line outside the park for what feels like an eternity.
As for getting there from offsite, don't forget to add in the time you will need to enter WDW. Not sure where you're staying, but from the first Welcome signs (if you were coming in on Rt. 536), the drive to MK is about 5 minutes. Then figure 5 more minutes to get through the MK parking gates and into your actual parking spot. Then either tram or walk to the TTC, which is 5-10 minutes. So don't forget about those 15 minutes from the front gate of WDW. Once you're at TTC, our experience is that the buses very rarely run, unless it's a holiday or some other strange reason. It's the ferry or monorail. I agree with Accident on this one. If the monorail is running, it's your best option. Look to see if the ferry is there and about to depart (you'll easily see it as you walk toward the monorail entrance, it's on your left). If it is, you can try doing that. But even if it is, I still think that unless the monorail line is ridiculously long, the monorail is faster. My wife and I have actually timed it, and the monorail is almost always quicker. You have to remember it takes the ferry a long time to load. Then they have to close the gates and untie the boat from the dock...pull it out slowly and then go. Then on the other side, they have to slowly pull into the dock, tie the boat to the dock, open the gates, etc... and if you aren't standing in the front of the line to get off the boat, that can take quite a while too. Monorail is your best bet most of the time.