Pedernales won't have much (if any) bluebonnets itself, but it is a lovely park. I'm not familiar with the RV camping facilities there, but I can say they probably aren't as abundant as you might expect.
Inks Lake State Park is where a lot of the RV and vehicle-based wildflower watchers go in spring. It has superb (and enormous) camping facilities -- it's the most popular state park in Texas -- and is an excellent basecamp for spring wildflower viewing. Unfortunately, you may have a hard time getting reservations for parks in that area this close to your intended visit. Visit the
Reserve America page for Inks Lake SP to check fees and availability.
Inks Lake provides reasonable access to McKinney Falls State Park, Inks Lake and the Marble Falls/Burnet area -- all good wildflower watching areas.
LBJ State Historical Park in Johnson City is also a good bet, but has no overnight camping facilities.
Be sure to check out the Texas Parks & Wildlife Department's Guide to Wildflowers & Wildflower Viewing!