Well what you have is a bad connection problem.
Some have had battery connection/terminal connection problems
Some have bad ignition switchs
Some have fuse terminals getting hot and the terminals spreading apart losing volts/amps
Some have had the connector that the ignition wires feed into do a melt down. Ect
So 8 volts might just light up a small light bulb but definitely will not work a relays ect .
I would say haveing an electrical problem is like having a big water hose and it has a big hole in it so you have to find the hole and fix it to get a lot of water out the correct end
Save yourself some time and buy a 12volt test light when you hit 8 volts it will be dim a must have tool
hth
Yes a multimeter will work and is great to use after/while verifying the problem.