Mike, I think you and all the folks who usually bring a trailer to the Park may want to read this. The policy is posted at the Park's website. Here's the link to the site and vehicle length limit policy. It's posted on the campground details page and says, "Sizes are for total length of vehicle and towed equipment." Here are the details for the Mammoth CG, specifically.
https://www.nps.gov/yell/planyourvisit/mammothhscg.htm
I knew that the Park was trying to add more campsites. In fact, last fall, I heard that the existing sites at Indian Creek were being shortened so more sites could be added for tent campers. But it looks to me like,this year, the Park may be going one step further, trying to force folks with a trailer to go outside the Park or force them to camp at the large RV campgrounds like Grant Village and Bridge Bay. It might be using the rule so the Park can get back to being a "tent-only" Park. I sent the Park's CS dept an email requesting clarification about the camp site/vehicle length limitation policy last week but haven't gotten a response yet. I'll post a reply when I get clarification of the policy for 2019. Here's what I know.
I first saw the site/rv length restriction at the Park's website either last year or the year before. I contacted the Park right away because our little 16' A-frame pop-up trailer plus our 17' SUV would put us over the maximum 30' length limit, not only at the Slough Creek CG, but also at most of the other Yellowstone campgrounds, as well. I really didn't want to have a bad ranger encounter over a miniscule 3', especially, since I was a senior who's been an annual Park visitor, camper and supporter since 1978. On the other hand, I also didn't want to be told that I'd have to leave SC and camp elsewhere because we were over the limit by 3 feet. When I made my initial Park inquiry about the rule (last year or the year before), I was told by the Park's lead CS ranger "NV"that we'd be OK if we parked our car next to our trailer provided that 1) there was enough room at the campsite, 2) we didn't block the campground road and 3) that our parked car didn't disturb the vegetation. So we parked our car next to our trailer and we didn't have a single problem. But I also know that the Park has a new Super this year, there's also a huge shortage of camp sites in the Park and that Park management personnel and the Park's rules and regs can change at any time. I also know from personal experience that there has always been a few "well intentioned" rangers, park planners and "bean counters" in the Park who have done some pretty silly things without thinking of the consequences, including making up a new Park rule and/or policy that, in the long run, does more harm than good.
Hopefully, the Park won't be strictly enforcing the RV/campsite/trailer length limit this year....but ya never know. To be safe, trailer and RV owners may want to consider contacting the Park to get clarification and the details about the policy before they come to the Park this year. The policy, the Park's phone number and email address for questions are posted at the Park's website under "Planning Your Visit" and the details appearing for each campground. IMHO, the Park should really clarify the site/ RV length limit rule ASAP so people aren't wasting their time and money, thinking that they'll be able camp in Yellowstone with a trailer or an RV. On the other hand, maybe that's the Park's intent...to get rid of all the big RV's and trailers and return Yellowstone to a more primitive tent-only national park. Not sure what the motivation or rationale was behind the length limit policy but I think that the Yellowstone visitors and supporters have a right to know.
Last fall, I heard that the existing sites at Indian Creek were being shortened to make room for adding some additional tent sites. Personally, I can't think of a single car/truck and trailer setup that would ever be able to meet the combined 30' site/vehicle length limit. I'll let ya know if we get kicked out of the Slough Creek CG in June because our car+ our little A-frame trailer exceeds the 30' length limit by 3 feet. If that happens, as a senior and as a long-time Park advocate and Yellowstone camper since 1978, I WILL NOT BE A HAPPY CAMPER! I suspect that many forum members like myself who own and/or camp with a trailer will not be happy either. So I hope you'll voice your concerns to the Park NOW and not wait. Thanks for listening and for allowing me to vent and voice my concerns about the new camp site/vehicle length limit policy. What do you think? Does the Park even need such a policy or is it just trying to lower the number of Park visitors because the Park is getting so over crowded.