I have no desire to change sensors or change the original tank on my Montana, so I can't help answer your question on these. However, one thing you should consider? If you are wanting a larger fresh water tank, you will most likely need larger black and grey holding tanks too. Unless you can dump on the ground.
It sounds like you are camping in areas where you do not have full hook-ups. If that is the case, the grey and black holding tanks have a limited capacity too.
It's just something to think about.
We camp at State Parks and, except for a few rare occasions, we've not had full hook-ups. We do at private campgrounds, but very, very few State Parks.
To get around the water "in" and water "out" problem, years ago I purchased a "Blue Tote" to haul our "waste water" to the dump station. There are some people who absolutely hate these things. I can't say I "love" dealing with my waste water, but I do "love" have the ability to dump without having to break camp and drag the camper to the dump station over 2 or 3 days. (our tanks are not that big either).
When we did state park camping (for years), we'd always attempt to pick a site that was within "garden hose length" of the common water spigot. That all changed and opened a new world for us when we purchased a water tank and a water transfer pump. The water tank came from TSC (Tractor Supply Company - 35 gallon) and the water transfer pump came from Lowe's. The water pump is 120 volt AC, but we also have a 400 wattt inverter we keep in the truck all the time. So if we do not have AC power for the water pump, we can always plug the inverter into the cigarette lighter and plug the pump into the inverter. It world great!
For us, carrying the 2 portable "tanks" (Barker Blue Tote) and 35 gallon fresh water tank, we are able to camp almost anywhere. Provided we still have a source for fresh water, and a dump station within a reasonable driving distance.
How I dump at home:
Fresh water tank:
My previous camper (Keystone), but works the same for the Montana:
