donles,
Look up the spa installation instructions online for that particular unit to see about the neutral requirements.Also my experience with spa dealers is they don't always know a lot. I had one exactly as you said, the GFCI would trip even with NO LOAD attached to it. The dealer tried to tell me that it was because the neutrals were not arranged right on the SAME busbar! He said the two load neutrals had to be on either side of the line neutral!!! Plus the unit had ran ok for a year before this anyway. Well after I finally argued another breaker out of him, everything was fine. Then he tells the customer the breaker could have been damaged by the neutrals not being arranged right on the bus.
Anyway you can get diagrams and wiring instructions online for most spas if you have the make and model.
