Welcome to Wild Texas, vettech2112!
I echo lost_but_found's "relatively flat" comment, although the exact route I took to the Wildnerness Camp did involve climbing up Cougar Rock and down a short, very steep section of trail (steep enough to remind me of what "skiing" in hiking boots is like!) just prior to the last trail junction near the site. I took that route precisely to get some elevation gain in hopes of photographing some of the parks vistas (and enjoy some of the prevailing winds atop Cougar Rock).
Glad the photos are generating interest in the trip, though I cannot believe
my trip was a year ago, now. I definitely need to get back into the saddle on my hiking and camping activities -- this is
embarassing! The views from the various trails should be better than I had, since wildfires in Mexico during my visit made for a persistent haze.
I see nothing wrong with biking to the campsite and then hiking from there -- whatever way is best for you to enjoy the outdoors is the way to go, IMHO. I documented in my HCSNA trip report that I was more than a bit envious of how much more of the park the equestrians I met got to explore, due to their mode of transport (horses & mules). In my mind, biking to the site is probably the best of both worlds, and allows you to scout around easier than someone on foot. For hikers worried about bikers missing out on the views or wildlife, don't worry, there are plenty of rocks and ruts at HCSNA to slow bikers down a bit (and hikers, too, for that matter!)
I don't know how well bike trailers handle off-road travel. Since I've always car camped (even if I have the soul of a backpacker waiting to leap out!), I have, thus far, always had the convenience of a cooler. However, having it seems to make me bring more than I need when I do that (even for two people -- too much water? Check. too many snacks? Check... etc.) That said, you'll want to carry enough water to be comfortable and hydrated -- that's a given even at the "developed" sites at HCSNA, since there are no water faucets/spigots in the park (except perhaps for the horses at Chapa's Group Camp). Horses can handle the nasties in water better than the human digestive system, I think...
Anyway, I'll shush up and let others chime in. You're heading to a site I want to return to for an overnight, and at a good time -- before the weather gets excessively hot. Don't be surprised if you don't need a sleeping bag, though, except as padding.
