Discounts or removal of charges.
If you're a Caravan Club member and present your card you only get charged as a car when towing a caravan or trailer. If you are very affluent and can afford a motorhome that is also just charged as a car. No idea on motorhome or trailer. I've no information on this from the M6 toll people but it is reliable.
If you are disabled and your vehicle is VED (Road Tax) exempt you can apply for a pass [You must be registered and logged in to see this link.] for free use.
This is much cheaper than the M6 toll and trailers are already free, or more accurately only charged at car only rates.
There is an exemption for vehicles exempted from road tax due to disability with as far as I understand no need to apply for a pass. It might be an idea to confirm this as new number plate charging will come into force soon. [You must be registered and logged in to see this link.]
I know of no discounts on the 12p to cross Warburton Bridge other than the all day 25p ticket.