It's 1.3 miles to the far east end of Main St. The majority of the shopping and eating you're probably going to want is further west. We usually enjoying eating at the Auslander and Hondo's. You might plug those into a Google map, just to get a feel for how far. Parking on a weekend can be difficult (not impossible), but if you can go during the week, it won't be as frustrating.

No, it is not and I do not know of ANY RV Parks in Fredericksburg that are within walking distance of downtown. Perhaps you could UBER.

No. If you look at Google Earth you'll see it's a good mile or so to E Main. If you like to walk, however....