Sounds like you live in one of the few 'unregulated' places in Texas.
You should be fine storing the tanks without water in them.
Composting toilets are not common, but it sounds like that would work in your circumstances. You might also consider an 'incineration toilet' which would reduce the waste considerably.
If you have no regulation, your grey water might be able to be emptied onto the ground using a hose and one of these.
https://www.amazon.com/Valterra-T102...10&sr=8-8&th=1
As far as maintenance, you need to pay special attention to roof seams because of the hot sun.
You also need to crawl under that rig and plug any holes with stainless steel wool to keep out the critters.
Another thing to consider would be an EMS/Surge protector to protect the electronic items in your rig.