In some cases, access to the drain piping and vent pipe can be accomplished inside one of the interior cabinets. Simply cut through the ABS piping, near the floor, using a hacksaw. When reinstalling, simply glue an ABS coupling to rejoin the pieces.
Keep in mind, once you assess the crack, that the only permanent repair to a plastic holding tank is by plastic welding. Patch kits, although readily available in the aftermarket, should be considered a temporary repair only. Only by plastic welding can a tank be permanently repaired, so look for a shop or dealer with experience using a plastic welding machine. If no shop in your area performs this service, I do have a Plastic Welding instructional videotape available if your shop is interested.
If the crack is severe or located in an area under stress, it may be more practical to replace the tank. You’ll have to contact your selling dealer or Keystone in order to obtain the correct, exact replacement. In some cases, holding tank replacements are better left to the pro technicians, but in some cases, an RV handyperson with adequate tools and an understanding of RV waste plumbing, repairs and replacements can be an alternate consideration.