The Isolator device might not have been defective....
If it sat for a while. with old enough batteries the batteries (house) may have been dead (run down dead, as well as past end of life)
Now many Intelletec Battery Control Systems use a BIRD (BI-Directional Relay) and with this system if the engine is running and the system voltage is high enough the batteries (All of them) charge, else only the engine
Well when it closes, thus connecting the very low house, the voltage drops and it re-isolates, then the voltage rises and it re-connects (After a delay in all cases) causing the surging you cited.
Also a breaker can open due to high load and cause it, then self reset, on some RV's. (Mine for example)
And of course the technician may have installed the new isolator wrong ---OR---
The converter may have blown a fuse due to dead house batteries or not been able to bring them up to the "Connect" voltage due to the house batteries being toast.