The advise on water capacity is a good point but what is important is determined by the individual and their camping style.
We have found our water and holding tanks last us a week or so. Since we rarely spend more than a 3 or 4 days in one place before we dump and refill, water capacity is rarely an issue.
Owning a TT is a three legged stool. Floor plan, tow vehicle and cost all have to be correct.
Floor plan includes the various TT amenities, a tow vehicle's max cargo is often the limiting factor rather than towing capacity and cost should not be onerous.
Also buying an RV is not a race, take the time to sleep on the information. Things often look different in the morning.