I am in the same boat as Pat from the stand point that I am the driver 97% of the time, so I am not sure what it’s like to ride in the back. My kids and my wife have never thrown up back there, so that’s a good sign.
To directly answer your question, YES there _is_ a limit to how much stuff you can pack in the coach. It is and always will be something you have to contend with when using a Class C motorhome that isn’t built on a huge truck frame (Freightliner or something of the like) like the Dynamax Force or DX3. You, as the owner/operator, have to decide what you are comfortable with in terms of packing into the coach. There are people who think the GVWR (Gross Vehicle Weight Rating) is the RULE and you do not exceed that no matter what. There are people who look at the GAWR (Gross Axle Weight Rating) of the vehicle and think that is the limit of the vehicle. There are people who think that is simply doesn’t matter and I’ll pack whatever the heck I want and I’ll just drive slower and more cautious. Make no mistake though, buying, owning and operating an RV build on a Class 5 truck chassis (Ram 5500, Ford F-550, etc) is a compromise between size and ability to carry weight. You ARE trading the ability to carry stuff for being in a RV that is smaller than the bigger Super C coaches.
I personally believe that loading the coach as “smartly” as possible helps A LOT too. Logical stuff like, don’t put heavy stuff up in the high cabinets. Keep the weight as low as possible. Don’t concentrate your load in one place in the coach. In other words, don’t put 4 cases of water and your gun safe (ha!) under the bed, but nothing up front. Spread it out. The other thing to remember is that the Isata 5 has a 10k pound tow rating, so if you are towing something behind you, load your heavy stuff in the towed vehicle/trailer.
As for driving with the tanks full, you are going to want to avoid that at all costs. The 36DS has a 90+ gallon fresh water tank. That is 720# of water and it’s going to be mainly loaded on the passenger side of the coach. We usually travel with around 10 gallons of water as a “just in case” measure for washing hands and flushing the toilet, etc and that has served us well. There is almost always a place to fill up at any campground we’ve stayed at. If we stay somewhere where we know there is not going to be water, we just fill up as close as we can, so we minimize the travel time with the tank full-ish. Trust me when I tell you that you KNOW IT when the tank is near full and you’re driving.