Can you have renal tubular acidosis with an elevated serum anion gap?

Possible: The textbook answer is no. Generally, RTA presents with a normal anion gap. However, if concurrent conditions of dyselectrolytemias exist the gap may be elevated.

Yes: RTA doesn't cause elevated anion gap, but other disorders that do can be present along with RTA. For example, advanced kidney failure can cause anion gap, and also cause RTA. Usually simple math will show that the anion gap doesn't completely account for the degree of acidosis, and urine tests can demonstrate presence of RTA.