don't need helper bag to tow a 18ft boat.: boat are light in the hitch. same thing goes for towing a car on trailer.
mulch weight about nothing.
Your other option are "helper spring" don't remember the name of the product exactly...but you can look at pretty much anyhting that is available for towing(other then airbag) and they can be fitted to a lowered truck with stock leaf
just need the one that fit your leaf wheter you have a flip kit or not.(diff over or under the leaf pack.)
they generally cost around the same as the helper bag.