Pour the juice from the pineapple chunks and the mandarin oranges into a measuring cup - you will need one cup total. You can combine the two juices or use one and substitute water to make a full cup.
In a medium saucepan, pour 1 cup of reserved juices. Add lemon juice to it. Whisk in the sugar and cornstarch.
Add the eggs to the juice mixture and whisk until combined and frothy.
Place the saucepan over medium heat and whisk constantly, cooking until the mixture thickens into a light custard.
Remove from heat, whisk in the butter, and allow to cool completely (about 1 hour).
Fold the whipped cream into the custard mixture.
In a large bowl, toss together drained pineapple chunks, drained mandarin oranges, grapes, marshmallows, and pecans. When the custard has cooled, pour it over the fruit mixture and toss to combine. Slice bananas and add them to the fruit mixture. Stir gently to ensure that the custard mixture coats the banana slices, but be careful not to mash up the banana.
Cover with plastic wrap, and place in the refrigerator overnight.