Steve - There is no exit from US 90/I-10 to Roosevelt. And you do not want to take I-37 south and exit at East Southcross Blvd. Southcross is where the extreme railroad crossing is located. So do not leave that direction either.
Now I am not sure exactly what is the best way. But without knowing any better, one option is arriving US 90 from the west, take I-35 south to Military Dr, then east to Roosevelt, then north to the RV park.
Second option would be from US 90 take I-410 around the southwest side and exit at Roosevelt, then north to the RV park. Probably the easier way.
Roosevelt is a good 4 lane street. And you will pass by one of the better restored missions and a Walmart.
I think you would have no problem either way. Maybe someone more familiar with the area could comment on it.