There are places to park a C, you might have to pay for multiple spaces though, and get there early. These will be within 10-12 blocks of Alamo, and connected by public transportation. Even in a car, we often cannot find parking within six blocks of the Alamo and Riverwalk entertainment area.
Most likely sites are west of Mercado, around I-10 and beyond, where the site of Fiesta becomes parking when not used for events. All I know of is paid parking, commercial lots more likely to take you for a price, rather than public lots letting you overflow a space. Public lots are also more time limited, commercial lots will sell all-day. Expect what you find downtown, willing to take a large vehicle, to not be self park. We often use a big lot under the freeway, don't know that I'd try to put a C there.
Other potential parking areas include lots around Alamodome, International Cultural Center, and Convention Center. A "trolley" styled bus for tourists connects most of these areas, on a limited schedule (hours not as long as most of the city bus routes). Availability at any of these places will depend on what is going on at the moment. Tour buses go to one or the other of these big lots after dropping off pasengers.
Other venues outside the downtown area have adequate parking: the other missions on the mission trail, most of the major museums.
Before my daughter lived there, I would put the RV in one of the several parks on a bus route, and ride downtown. While my daughter was there, we would do day trips from her home, It takes 2-3 days just to catch highlights of SA. Riverwalk in particular is a different experience at different times of the day, different days of the week. It can be morning or afternoon tourists, a noon business lunch venue, or focus of nightllife for affluent citizens and businees travelers on generous expense accounts.
Alamo and the oter missions are best visited by day, but to really see them you probably shouldn't plan for more than two mission visits per day.
There is almost always something going on somewhere in SA, so be sure to pick up a schedule of events, to plan the timing of your visit, and what you might do while you are there.
