First drain and flush the tank numerous times (it had to be the black tank, right?), and then begin filling the tank with fresh water. A garden hose into the toilet will probably be the quickest method though speed is not the point. As the tank nears capacity, closely look with that mirror and flashlight to see exactly where the leak appears first. It's common to see cracks on the sides of the tank near the top; rarer are cracks on the very top of the tank. It could simply be leaking from around the 3-inch pipe from the toilet or the smaller vent pipe requiring nothing more than new sealant.
If a vent pipe were to drop it usually drops into the tank through a rubber tank fitting. This may or may not cause an actual tank leak. You can check the vent pipe from up on the roof. Remove the sewer cap for that vent line and look for the ABS pipe. It should stick up about an inch above the roof. If it is much lower than the roof line or not visible at all, then it may have indeed dropped into the tank.
But I digress….If it's not possible to view the top portion of the holding tank from below the motorhome, then yes, the tank will have to be dropped. It may be suspended by hangars or angle brackets. It's not a difficult task though it is time consuming to remove the holding tank.
First remove the toilet and the floor flange as well as the vent pipe that extends up through the roof. It may be necessary to cut the ABS vent pipe close to the floor line and use a coupler fitting after reinstallation. It is also necessary to remove the 3-inch tank outlet fitting and to disassemble the termination assembly, thereby isolating just that tank. It seems like a lot of work, however, it is simply a matter of logically taking apart a puzzle. After the tank has been removed, reinstall the closed termination valve to block the outlet, support the tank and refill it while on the ground. It will soon be evident where the leak exists. Or the crack may be quite visible after dropping the tank.
Plastic holding tanks can be successfully welded using a plastic welding machine, but not all repair shops are equipped with a plastic welder so you may have to shop around. Avoid using an aftermarket patch kit. The only viable method of repairing cracks in plastic tanks is by welding. Patch kits can, however, be used in an emergency situation or for a temporary repair. Keep in mind some repairs may just not be feasible; existence of numerous cracks, deteriorated plastic, impact damage, etc. In those extreme cases a new tank is in order. Always install the largest possible tank that will safely fit in that same area.