number 1. only use fuses of the same rating. they are there for a reason.
number 2. the fuse should have nothing to do with the voltage loss when cranking.
put a 20A back in there, and try it out again. if you are upping the fuse rating, you are running the risk of pulling more current than the manufacturer designed the circuit for.
i dont know what caused the fuse to pop in the first place, but when you put the 30A in after that, the same event may have occured, and since you allowed more current to pass to the equipment, something may have been damaged. like i said, put in a replacement 20A and try your system how it was running before.