Don't try to take your trailer to the visitors center. Almost impossible. Agree with the comment about the 7th Ranch RV park. Stayed for two night this spring. Nice place and three miles from the battlefield. If only stopping on the way somewhere, there is plenty of parking just down the hill at the freeway exit. A casino parking lot and room along the roads to drop your trailer. Highly recommend the film and the Ranger talk. Great background and history refresher.
Also bought the CD and did the car tour (Buy and the gift shop). Very well done, allows you to start and stop the CD at points of interest along the 6 mile road loop. You can stop and get out to walk the area. (some places but not all - watch the signs) I thought I knew the history pretty well, but actually being there and seeing first hand what the doomed unit saw was amazingly well done. One of the best national parks/monuments I have ever been to.
As far as depressing, I guess all battlefields give you that feeling to some extent, but no reason to deprive yourself of the learning and immersive experience that this monument provides. Very well worth the time to visit