Reservations right now will definitely be a problem but check anyway. You didn't say what size your fiver is. For us, we would not stay in the park due to the size of ours (35' 5') unless we could get an end site in Madison so that we wouldn't have to drive around a loop to get to it (internal loop roads are tight then add units sticking into it and you've got a problem), or Bridge Bay in the open area up front. Bridge bay takes a lot of motorhomes and yes they allow generators. Some of the Bridge Bay sites in the back can accommodate big fifth wheels in back in spots. As for outside the park and you're coming from Livingston - you'll be coming in the Gardner North Entrance. Check if you can get a spot at the CG at Mammoth there. If you were coming in through Bozeman, I would highly recommend Baker's Hole FS CG just 2 miles North of West Yellowstone, MT. It's a first come first serve but well worth a little planning to get in. It's a beautiful FS CG on the Madison River, scenic, and big rig friendly. Most sites have electricity and there is water at the entrance to fillup and if you miss it like we did, there are threadless faucets throughout that with a "water thief" you can tap into, which we did. What we do to get into Baker's Hole is first drive in and check if we can get a spot. If not, we go into town and stay at an RV park for the night - we prefer Grizzly but any will do. Plus if you stay at an RV park, you can fill up your water and dump your tanks. Then the next morning we drive our truck over to the CG and get a site, pay for it then go back and get our rig. We always check in with the hosts when doing this so they know we are not saving a spot per say. They actually told us to do this when we stopped and talked to them about available spots coming open after driving through and not finding one. Flagg Ranch and Colter Bay campgrounds (there are two - RV hookups ($$) and dry ($) are also good choices but much, much further from Yellowstone than Grizzly or Baker's Hole.
