new RV driver - need info about roads and sites
Will,
From your response I am presuming that you will be most comfortable with the least mountainous route. I am going to suggest 2 routes in and one out. The first has only the mountains at the East Gate of Yellowstone and the second has more challenges. Keep in mind that the scenery is directly proportional to the amount of mountains. More mountains, more great things to see!
I preface everything by saying that all the roads I suggest here are relatively wide, sparsely traveled (by east coast standards) and have moderate grades and few sharp turns. I'm presuming that you already have the Black Hills section of your trip planned so I won't go into that.
The least mountainous route from Rapid City is as follows: South out of Rapid City on SD79 to US18. US18 West to Mule Creek Jct. US18/US85 South to Lusk, WY. US20 West to Orin Jct, WY. Go North on I25 (labeled north but the road is actually running east and west here) to Casper, WY. At Casper continue west on US 20 to Shoshone, WY. In Shoshone turn right to continue on US20 to Thermopolis, WY. The road from Shoshone (say "show-show'-nee") goes down through a spectacular canyon. The grade is not steep and the road is wide and curves gentle. In Thermopolis turn left to go west/northwest on WY120 to Cody, WY. In Cody pickup US20 again and go west into Yellowstone. Your first really good grade is the pull up out of Thermopolis. I wouldn't call it mountains but it is a good grade that goes on for a long time. The next challenge is US 20 from Cody to Fishing Bridge in the Park. This is a mountain road but has been widened and straightened in recent years. It is definitely no worse that some of the Smokey Mountain roads back east.
A more challenging but more scenic route is West on I90 from Rapid City to Buffalo, WY. In Buffalo take US16 to Worland. Be prepared, this is a real mountain road. The road climbs about 5000 ft from Buffalo to the top of Powder River Pass and then drops about 5000 ft to Worland, WY. It is steep on both sides, the west side is the steepest and has the sharpest curves (and is the prettiest). From Worland go to Thermopolis and on to Yellowstone.
I strongly suggest that you have reservations for your campground in Yellowstone. Go to
http://www.nps.gov/yell/pphtml/camping.html. I might suggest Canyon or Madison Campgrounds. They are both centrally located and minimize travelling to see the sights. If you are most interested in Yellowstone Lake/River things stay in Canyon. If thermal features and wildlife are more interesting to you then go on to Madison. Either way you can day trip in your truck anywhere in the park. The internal park roads are formed in a rough figure 8. The northern loop has places where the road is in poor condition, steep and winding. The Southern Loop roads tend to be much wider, smoother and less winding. All can be negotiated with your rig but you probably won't want to, especially the road from Canyon through Tower to Mammoth.
I'll take this opportunity to give some advice on mountain driving. I don't believe that your rig will have ANY problems if it is driven appropriately. Pulling grades in a lower gear with higher RPM's (less than redline) at part throttle is less stressful on the engine and transmission than a higher gear and lower RPM's. You won't hurt your engine if you are pulling these grades at 4600 RPM. You might hurt it if you pull for MILES at full throttle and 2000 RPM. This is the reason that Ford recommends that you take the truck out of overdrive while towing. You need to gear down manually to second or first if the grade is steep enough. If you are full throttle or near full throttle and the RPM's are low shift'er down! NEVER let the transmission 'hunt' for the right gear. If it is doing that then shift it down until the grade gets less and it pulls willingly in the next gear.
On the down hill side your truck and trailer brakes are inadequate to keep you at a safe speed on the LONG, STEEP grades. The gasoline engine in your truck however is an excellent brake for this purpose. Manually shift the transmission to a lower gear until you find the one that holds you at a safe speed with minimal or no braking. If you must brake (this will get pot shots from some who prefer another technique) apply the brake with just enough pedal pressure to hold your speed or reduce it for corners. Avoid heavy braking if possible.
I presume that the automatic transmission in my 32' motorhome is too stupid to know when to shift so I ALWAYS shift manually. By being the brain for my transmission I avoid situations where the engine, transmission or brakes are overstressed. It is twice as heavy as your rig and has 1/3 bigger engine. It has NO problems on any of these roads. You and your truck won't either if you do it right! ;-)
Unless you have some reason to go to Idaho I would suggest that you avoid I80 in Wyoming. This road traverses a high rolling plain. It has endless 3-4% up and down grades. The wind always blows! Truck traffic comprises 75-80% of the volume and the trucks run 90+ MPH on the downhill sides of these grades and 30-70 on the uphill sides. With your rig you will be continually passing slow trucks going up and being past by 90 MPH juggernauts on the way down. You haven't fully lived until you have had an 83K lb eighteen wheeler pass you at 90 MPH while you are going 60!! ;-(
I suggest that you exit Yellowstone south to the Tetons. Spend some time there if you can. Leave the Teton NP going east on US26. This is a mountain road but by now you will be experienced ;-). It is not nearly as entertaining (steep, winding) as US16 from Buffalo to Worland. Take US26 to Shoshone and US 20 on to Casper. From Casper find your way home. Either back the way you came or south to Cheyenne and east on I80 or on south to Denver and east on I70. I70 East to St. Louis is OK, I have no opinion on I80 in Nebraska.