RE Holding tank: A good rule of thumb that I use after tracking trip data for many years (yes, I really do this!) is you use 1 gallon of tank space per day per person. So a 100-gallon tank will last 10 people 10 days, or 2 people 50 days during the summer months. This analysis is for drop-through or efficient flush heads (i.e. Vacuflush), not regular house-type flush like are on many of the newer private houseboats. So for two of you, if the tank is 150 gallons, you may not need to pump out prior to returning.
For winter trips, ave use is 1.25 - 1.50 gallons per day per person.