We just visited St Louis last spring. Pulled our 35 foot camper. We stayed at the St Louis West KOA. It worked out just fine for us.
Staying at the Casino didn't look like the best part of town, so we opted to drive 20 minutes into town each day. It was a non issue. This is also very close to 6 flags, if you plan on visiting.
Don't miss City Museum. We underestimated how long we'd need to stay there. We were there for 4 hours and closed the place at midnight, and my youngest was only 8! It still wasn't enough time. Probably not the best place for toddlers, but for school aged to adult, it was a blast.
As far as navigating the city goes, we drove from New Orleans to St Louis KOA without issue and pulled in at 10pm. 2 days later, we left St Louis at 9am for Michigan, and had no issues either way.
On a side note, I do not recommend visiting the zoo. We went because we had heard great things about it. It's free, so 6 million other people will be there with you. I did not find it to be superior to the local big city zoos in Detroit or Toledo. We could have spent our time elsewhere.
Enjoy your trip!