Although haven't played much trem, but a different yet related game, camping seems a problem with every game of these characteristics.
The origin of camping is that by staying safe in base, the attackers are forced to fight both players and defenses.
Attackers get weakened with every attack, while defenders gain strenght because of lower loses during defense.
This can be however broken easily by forcing defenders to move out of their base by placing outside the defense's range something that if left alone would increase attacker's strength over time.
For this goal, special buildables that behave differently from normal structures can be used.
Key elements to ensure this can only be use for attacking are:
Timing:
First they have to be built near near attack routes, then activated which also takes some time to prepare until it can provide the effect to nearby players.
Only those initiating the attack can easily time it, and even in case someone manages to time them right for defending, attackers can retreat temporarily, wait for the effect to wear off and resume attack afterwards.
Defenders retreating to avoid fighting the powered up attackers would sacrifice the base when retreating, thus are forced to face them.Mobility:
Once the effect has been activated, moving and time wear off the effect. For chasing players that can retreat, its usefullness is limited,
however if players are unable to run away, as happens when bound by structures, this limitation doesn't have that much impact on attackers.
The only possible countermeasure for those powerups is to prevent the other team from building them in the attack routes, which means leaving the base and attacking instead.
Powerup buildables also have to be single use, so no matter what outcome, you need to start from scratch to use them again.
Also, unlike normal structures, they have to cause negative side-effects when there are nearby certain friendly structures, to make it further harder to use on defense.
Otherwise a camping team could always prepare a powerup in base to use in in case the other team is building their powerups.
One simple yet effective side effect is that their activation hurts/kills and/or temporaily disables nearby friendly structures, so activating them for defense would replace the bonus of structures with the one of the buildable, but not stack with it.
A proposal of possible powerup buildables:
For aliens, once activated, the buildable grants a special armour to any friendly player touching it.
Time active should be long enough so all players have a chance to touch it before attacking, but not too long so players cant renew it once it weared off (unless building more than one, and activating them at intervals to time one per attack wave)
The armour decays over time, with movement and damage taken. By reducing the damage taken, the time attackers can stay causing damage before retreating or dying is increased, thus overall damage done is indirectly increased.
Also, as they start strong and weaken over time, when facing ranged weapons, by the time they cant yet reachs the humans is also the time they take less damage.For humans, it would work the other way around, by increasing damage done directly.
For long range attacks, doing the most damage before the enemy gets close also allows humans to keep themselves alive during the attack.
This powerup could take shape in form of special ammo for weapons which weakens over a short time until ammo becomes normal, or by
special weapons that are weaken the further from the buildable that provides them.
edit:
Edited so much above before posting that forgot some part of it:
This is part of some random thoughts talked with Ishq on IRC. Probably has a lot of flaws still, but may be at least can help finding a way to solve camp without having too many side effects in gameplay. Or worst case, an example of something that doesnt work. In any case hope this idea can be helpful for game development in some way.