You should really focus on strength training between competitions, not during "fight camps". The time to get big and strong is not when you should be developing skills specific to your sport.
With that said, that's not saying you should just forget lifting, but it definitely should take a back seat until after your fight. If you want to maintain your muscle while you train just make sure to eat enough protein.
To go ahead and touch on lighter vs. heavier loads during lifting, "heavy" is really relative to your current goal. You should by no means be trying to do 1 rep maxes every time you lift; as the first responder to your post mentioned, it would create CNS fatigue. Instead I would aim for reps of 5-8; explode on your lighter sets (performance prep lifts), and then focus on maintaining bar speed and explosiveness on your heavier weights (performance lifts) regardless if the bar slows down. Terminate once bar speed decreases significantly (before complete failure).
For example, let's say you start with 95lbs on bench press and want to do a max set of 6 reps (weight/reps):
This is a good way to maintain explosiveness and strength without worrying about CNS fatigue. Just remember that it's not as important as your sport-specific training. Pay attention to your performance on the mat when you change things up in the gym and you'll know what works and what doesn't.