As a twice daily user of the dartford crossing I suffer most days, fortunately I cross contrary to the worst traffic. the delays at the tunnels nowadays are usually the queues to exit at junctions 31 and 30. most accidents are at junction 31 where drivers (contrary to the signs) use the right tunnel, then cross the solid white lines to exit junction 31, some crossing from the extreme right lane - crossing 4 lanes of traffic - there are only a few hundred metres to cross in, most are taken out by lorries that cant stop when a car cuts in front (I have seen a few cars there sideways across the front end of a lorry). junction 30 queues back onto the m25, sometimes back to the tunnel.
As for a toll - its not a toll anymore its a congestion charge, when it was introduced it was carefully worded to include tunnels over a certain length - which made the tunnel eligable.
the proposed new tunnel will run further east near gravesend, which is good as one proposal was to put in another tunnel at dartford, this doesnt move the traffic away from the approach roads, gravesend makes sense as all the m2 traffic will use this, halving the traffic through the existing tunnels, it will exit between junctions 29 and 30. I travel up the m2/a2 daily, and half the delays are between where the new tunnel will turn off and the m25.
one lad at work recently got caught - he went through and back, but only paid one toll, not realising, they let him off with just paying the missing toll as he had not realised you had to pay each way.
with the pre-pay service you can put a certain amount on the account, and it will email you a reminder when it drops below £10 credit, you can also put all of your vehicles on the one account and the credit is there until used up, so as you update vehicles you can add the new one and take off the old one. well worth it as you get a 30% discount for pre pay accounts.