We are planning a western trip in late August thru September and would like to hit Yellowstone just after Labor Day coming over from South Dakota. From there we would like to see the National Parlks in Utah and then work our way back east from there leaving the area by the end of September. We are in a 2005 Tiffin gasser 35'. Are there any routes recommended (or to absolutely avoid) due to grade issues?

I would get the app or books called Mountain Directory, there is an East and West version. That shows you the steeper roads. I would mostly avoid any scenic drive on a secondary road with the RV unless your are experienced in mountain driving. Use your toad for touring.

Recently we were taking a secondary road that had lots of curves and steep up and downs. Totally impressed by the gasser on how well it did. We have experience in mountain driving so once I confirmed from others that the road was OK for our rig we didn't hesitate.

Good luck.

We traveled the country for 12 years in a 35' motorhome powered by the early Ford Triton V-10 and towed a CRV nearly every mile. In that time we crossed the country on pretty much every major route and never once has any problems. We traveled to Yellowstone and through it several times, to Devil's Tower and Mt. Rushmore as well as Custer State Park. As long as your Tiffin is in good mechanical condition, all hoses and bests are in good shape, and all fluids current you will have no problem. If you have not done so, it might be a good idea to flush the cooling system and the brake system and replace the fluids before you go. 

I recall that Ford switched from the 4R100 4-speed transmission to the 5R110W 5-speed transmission with tow/haul mode sometime around 2005. Does your motorhome have the "tow/haul" mode button on the end of the shifter stalk, or the "overdrive" button. 

I ask because the tow/haul mode is a huge help when descending mountain grades.  So if you have it, use it for sure. If you don't have it watch your speed carefully on the descents and be ready to manually downshift to help control your speed. 

During our travels we've seen many gas motorhomes of all ages doing just fine on the mountain roads.  The biggest issue is how well you know how to come down a hill without using your brakes constantly.  Downshift before you even begin the downhill; not in the midst of the downhill.  Going up you'll just be going slow.... and you'll have company.  Best not to pull off and stop when going uphill because you'll have a difficult time getting power.  If you're on an interstate just stay in the trucker's lane. They'll be going slow.  The best scenery and awesome camping is had by driving those kinds of roads.  You'll soon get the hang of it. 

I'd also recommend that you get the 'Mountain Directory for Truckers and RVers'.  Even though we loved driving those roads we always referred back to the directory when we drove it again... just to refresh us of what's coming up.  6&7% grades are very doable.  Some interstate sections even have those grades - and higher, such as I-70.

+1 on Tow Haul if you have it, game changer on downhills.  Don't ride brakes (also as mentioned above) hit them hard, slow down, stay off of them for 45 seconds if possible.