A little electrical theory.... Solenoids convert electrical current into motion (of a rod). Remember the experiment you did in High School where you ran a magnet through an electric coil to generate electricity? Well a solenoid is that in opposite. Current in, motion out 
Solenoid
Industrial Work Solenoid Construction and Basic Operation
from the latter article, the important info is:
"The force generated by a industrial work solenoid is dependent upon the current flowing though the coil windings. The average current in the coil of a DC industrial work solenoid is the simple result of average source voltage divided by the resistance of the coil."
So, although the service tech did not likely know the theory, he is "sort of" right. Adding a lot of accessories "sucks down" the voltage available and if it gets to low, and the alternator cannot keep it up, there may not be enough juice for the solenoid to operate consistently if the ground is poor. Hence BRP's solution to address the issue.
Also, with that little lesson in mind, it is VERY possible the new solenoids have a "wired" ground to make them more electrically efficient (because they are not depending on a "floating ground" which comes from the mechanical mounting only) and so they operate when you want them to..... floating grounds are not there there 100% of the time
I'd carry in printouts of the posts (or email him links), and gently nudge your tech on the fix..... he'll find it, and fix it.
I hate to say it, but a higher output stator, something that folks do when more accessories (specially high watt radios and CB's) are added, may be the end all solution, and one everyone hopes is not needed. An old motorcycle trick to prevent stator burnout is to solder the stator output led into the harness instead off depending on the plug/spade/whatever connector. Over time corrosion can affect the quality of the connection causing the stator to burn out. But that's years down the road.
Solenoid
Industrial Work Solenoid Construction and Basic Operation
from the latter article, the important info is:
"The force generated by a industrial work solenoid is dependent upon the current flowing though the coil windings. The average current in the coil of a DC industrial work solenoid is the simple result of average source voltage divided by the resistance of the coil."
So, although the service tech did not likely know the theory, he is "sort of" right. Adding a lot of accessories "sucks down" the voltage available and if it gets to low, and the alternator cannot keep it up, there may not be enough juice for the solenoid to operate consistently if the ground is poor. Hence BRP's solution to address the issue.
Also, with that little lesson in mind, it is VERY possible the new solenoids have a "wired" ground to make them more electrically efficient (because they are not depending on a "floating ground" which comes from the mechanical mounting only) and so they operate when you want them to..... floating grounds are not there there 100% of the time
I'd carry in printouts of the posts (or email him links), and gently nudge your tech on the fix..... he'll find it, and fix it
. I hate to say it, but a higher output stator, something that folks do when more accessories (specially high watt radios and CB's) are added, may be the end all solution, and one everyone hopes is not needed. An old motorcycle trick to prevent stator burnout is to solder the stator output led into the harness instead off depending on the plug/spade/whatever connector. Over time corrosion can affect the quality of the connection causing the stator to burn out. But that's years down the road.
