I am assuming that you are going to add one of the swing away hand rails, since the 210WBS came with a grab bar in 2008...
The instructions for the handrail should have instructions for the size drill to use, but, in the event it doesn't, when using screws in fiberglass, aim for a bit that is 1/64" (1/16" if in doubt) larger than the screw's shank. With fiberglass, the main support comes from the adhesive/sealant that should be liberally applied under the mounting bracket and the screw itself is used to pull the base down snug. By allowing 1/64" over the shank size, you allow for some expansion and contraction of the shank and should prevent any fiberglass damage. Aiming for a stud would also be advantageous...if you use the grab handle as a guide (it should be in a stud), tape a vertical line to align the new brackets...
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)