Rushmore routing

first thing to know: The Sturgis Bike Rally will be held from Aug 4 through 13, which means a whole heck of a lot of motorcycles will be filling up the Black Hills campgrounds and motels, etc.
I don't know of any boondock places. Since you're coming from the west on I 90,
I suggest you take the Moorcroft exit (highway 16) and go to Custer, SD. Then continue west on 16 to Custer State Park and drive through the park to other side where there is a nice park vistor center, not big but big enough to park for awhile. It is just past Game Lodge campground, on the left side of the road.
If time permits, ask if you can unhook and take your truck up 16A (Iron Mountain Road)... it's a beautiful drive.
Leaving the park, continue west to highway 36 and then go north on 79 to Rapid City. Flying J truck stop on 79 about a mile north of junction 36/79.

P.S. I see you want to keep your trailer hooked up. I know that Rushmore has parking for some RVs, but don't know if there is a limit on the size. Call and ask them. That would be your best bet.

The route you have listed might be doable with your 5er, but I wouldn't take mine through there without driving it first. There can be a lot of low hanging trees.

TexasShadow had good suggestions for you.

you can get to Rushmore easily if you go into Custer and take 385 north to the road to Rushmore. I think it is 244. then you can continue east from Rushmore and go through Keystone and on up to Rapid City that way.

Parking for RVs at Rushmore is tight to put it mildly. I was there last summer and there is only one road in and out of your hooked up. Iron Mtn Rd and Needles Hwy are both no go's while hooked up to anything.

Looking at your route, you maybe better served by taking I-90 to the north side of the Black Hills and staying in Spearfish. Chris's Camp is a nice place, great owners, easy on/off the interstate.
Spend two nights, take a day trip down HY 385. 385 to Keystone/ Mt Rushmore, then 16A(Iron Mountain Road) great winding road thru the hills with nice views of the Presidents. 16A to Playhouse Road(753) to 87(Needles Road, a must do) up thru Sylvan Lake to 385. 385 back thru Deadwood and back to Spearfish.If you want to see the Crazy Horse Memorial go south on 385 about 10 miles from the intersection of 385 and 87. About 170 mile trip, easy 1 day drive.
Great day trip with no 5th wheel. 16A and needles are best done with just the TV. Keystone is a nice tourist town. Deadwood has casinos with good eatery's. Go see Boot Hill and a few other things.
When you leave you may want to go to ND by of US 85 to Belfield and then 94 east. Better, faster road, less intersections and stop signs, and a town, Bowman, then the route are planning. It is about a hundred miles from Belle Fourche to Bowman. Longest stretch without a town.
If you have time stop by Devils Tower, adds 25 miles to your trip but well worth it. There is a KOA there and a National Park campground (no hookups) but nice for a night.
Have a good trip it is a nice area, but make campground reservations.

Co-worker went last year, with his 30 ft Avion TT and Loooong Dodge 3500, and stayed at Center Lake Campground. It is "same day reservations" basically walk up only, so there will be sites open even in the busiest times. No electricity. $19 per night, plus a $20 State Park fee good for 1-7 days. Vault toilets and showers.

http://gfp.sd.gov/state-parks/directory/custer/campgrounds/center-lake/

Just stay off of Iron Mountain Rd and Needles Highway with a Motorhome or trailer, due to tight tunnels and switchbacks.

Also be aware of the total solar eclipse on August 21st, and its path. Campgrounds and such are very booked up along the path of it and either side of the path, so plan your travels carefully.

http://xjubier.free.fr/en/site_pages/solar_eclipses/TSE_2017_GoogleMapFull.html

Charles

You guys are great, thanks for the info.