Three night church revival in a tent out back: do we still need an ADA porta potty if the building is open?

Yes, you need an ADA-compliant portable toilet for the tent area even though the church building is open. The 2010 ADA Standards require that where portable toilets are provided in a cluster, at least 5% (practically, at least one) must be an ADA-compliant portable toilet. Because you are providing a cluster of units at the tent, the ADA's 5 percent cluster rule applies. The availability of the indoor restrooms does not exempt the temporary facilities from this requirement, especially since the path to the building includes gravel and steps which are not wheelchair accessible.

For a crowd of 150 people over three hours, two standard units are likely insufficient. The industry guideline for a four-hour event starts at about one unit per 50 to 75 people. With a potluck and coffee, which increase usage, you should consider adding more units. Two standard units plus one ADA unit would be a better minimum. The building restrooms will help, but the distance and steps will discourage use. For a three-night event, you will need at least one service visit during the event to pump the tanks and restock supplies, as three nights of heavy use will fill the holding tanks of standard units.