Keeping water in the toilet bowl is so that there is a VAPOR Barrier .....
Residential toilets have built in 'p' trap designed into bowl fixture.
RV toilets do not..straight gravity dump so water in the bowl does keep seal wet but is for keeping the stink from waste tank coming back up thru toilet
Simple.....
As for weight carrying water.....
If the weight of a full fresh water tank causes issues (being overweight/pushing limits of tow vehicle, etc) then need to rethink choice of tow vehicle and or how much stuff you have in trailer.
Frontal area and height of trailer have more effect of MPG then weight does.
Besides...what are you going to do when you show up and there is NO water available due to issues with CGs water system or you have to hunker down somewhere unplanned due to traffic/accidents/fire etc that can change ones whole plans.
Carry the water, poop in the toilet, use the RV and don't make it so complicated.