Insurance company won't see it that way. Nobody is going to be weighing caravan at crash scene. When investigating claim few days or weeks later the only thing they have to go off is MTPLM and vehicles max towing limit.
This is incorrect. The insurer would have to prove the weight, this is often going to be almost impossible, although in this case would certainly be looked at and would it would normally need a specific policy condition to cover this. Some do have one limiting the trailer to the kerb weight of the car, but many do not. so you need to check your policy.
If the policy limits the trailers weight the insurer would have to prove the point which after an accident is generally impossible. Once a caravan has had an accident and the contents are all over the road or it has been driven home if the damage was less there is no way of proving the weight. In general the only times they could could do this is where the MTPLM exceeds the kerb weight or is so close to it that it is near impossible to load it below the kerb weight.
If the condition was not there it would have to be shown that the combination was dangerous, which would be a far higher standard. Given the conflicting opinions on towing ratios and the like it would need you to have done something silly to prove this. Possibly other motorists saying you were speeding and the caravan swaying badly might meet the criteria.
In both cases though if the insurer was unhappy with what they found it could result in terms being applied at renewal or a refusal to renew, which would certainly cost you in the long run.