There is, but I doubt you'd like it. West to Sarnia and down I-75 takes you around the worst of the hills, but from Cobourg that's quite a piece out of your way.
My suggestion would be to either take a short 'experimental' trip first on a route that will take you up & down some good-sized hills, or just take it easy and enjoy the trip south.
I doubt you"ll have any problems, but you won't be breaking any speed limits on the up hills and learn you'll to go over the tops a little slower so you don't have to ride the brakes down either. You won't be setting new fuel economy records either.
I have a smaller class C, but my dad has basically the same coach as you. He swears the extra 250km, 150 miles (in his case) to go out through Sarnia and down I-75 is nothing compared to the extra fuel burned going up & down all those hills on I-77 / I-79. He says to him it's a lot more relaxing too. Having done both routes many times myself, I find I-75 a lot more interesting, there isn't the long, long stretches of nothing but fields & trees there is going the other route.
If time is no big issue, you could also go out past Kingston and cross at the 1,000 islands and go down I-81 to Washington, D.C. and then all the way down I-95. It's a more 'urban' route but offers some great side-tracking along the way. Out to the Delaware coast, the Chesapeake Bay bridge/tunnel is great, the OBX (Outer Barrier Islands), Myrtle Beach, Charleston, Savannah are all great scenery.
Ted 'n' Laurie, plus Jackson (aka Deputy Dog, the Parson Russell Terrier 'fur kid') and, Rylie (who crossed the Rainbow Bridge June 14, 2012).