2007 Tioga 31m towing capacity

The OP'S MH is 32' long on an E450

Tow ratings seem to be all over the map for Class C motorhomes.

Our 2005 Itasca built on a 2005 Ford E450 chassis has a 5000 lb. towing capacity, apparently using the Ford frame extension and hitch as installed by Winnebago.

I wonder if the tow ratings from Class manufacturers vary by whether it's an E350 chassis or an E450 chassis?

The OP didn't say which chassis was under their 27 footer. I have seen what looked like up to 28 foot Class C motorhomes built on an E350 chassis in the past!

Probably the hitch mounting and/or frame rail extensions were reinforced or otherwise are stronger on the newer models.

Mounting the hitch on a class C RV is not the easiest thing in the world because of the very long distance between the axle and other original chassis components and the back of the vehicle; it makes a nice long (and hence powerful) lever to amplify many of the trailer forces. That's undoubtedly somewhat of a simplification of the actual issues, but it gives the general idea.

Are you sure the 2008 had the same GCWR ? In 2008 Ford increased the GCWR by 2,000#. That 2007 E450 when it left Ford, had a 14,050# GVWR and a 20,000# GCWR. After Tioga stretched the frame and put their house on it, they downgraded that GCWR figure. In 2008 Ford changed the specs to 14,500# and 22,000# respectively. So Tioga upgraded the GCWR somewhat, but it's still considerably less than what it was when it left Ford. It's only in the last year or so that SOME manufacturers are rating them at the full Ford rating.
If you have a 3,500# rated hitch, you definitely want to upgrade that to probably a 7,500# hitch or so, and have a qualified hitch builder/installer check out the frame extension and reinforce it if necessary. Another issue would be tongue weight. With a 4,500# boat if you can keep the tongue weight around 7% (315#) or so, it should be sufficient tongue weight to trailer properly and not be excessive tongue weight.