Look at the thread titled "Block Heater Tripping GFCI".
Never seen an installation like what you described, it may violate 400.7 or 400.8. that aside for a moment, I would make a note of what buses trip on what receptacles. if it is all of them, I would look at the most likely connection point to cause the ground fault trip which is where the plug of the engine block heater goes into the receptacle on the cord end. Other places to check are the plugs on the block heaters themselves, look for abrasions or corrosion. I would also check the panel for corrosion and look for any signs of water infiltration or corrosion on the terminals.
While going to GFCI receptacles would not shorten the wiring run any in your case, it would allow you to use a factory extension cords with molded factory ends on them which are in my opinion more water resistant than any field-made cord. How old are those cords? If the ends have ever been run over by a bus, or the bus moved while the engine block heater was still plugged in, there could be damage to the fine stranded conductors within the cord.
Please post any problems and solutions you find with your installation. The temptation to use GFEP breakers over GFCI is there, though there is the possibility that even those will trip, and then what? Yank them out and put in regular breakers? Neither solution is allowed per the NEC.
Good luck.