When we used our camper last winter, we went through one 30 pound tank of propane in 6 days heating the camper (and we supplemented with 2 electric ceramic space heaters). When the temperatures finally hit zero degrees outside, we moved back into the house. Of course, we ran no water heater and fridge ran on electric.
Fast forward. Now it's Summer. The outside temperatures are averaging 75-85 (day and night) for weeks now. We are still in the camper 24x7. But now we have the water heater on, water running, use the oven and stove some, refrigerator still on electric, and no furnace because the Air conditioner runs on electric all the time. Propane usage? Refrigerator runs on gas when we are traveling, and we've been on the road a lot this year already. Once at the camp site, it flips back over to electric.
Well, I filled the 30 pound tank last time in March and we STILL have plenty of hot water! Returned from a 16 day trip to South Dakota, and thought for sure I'd be flipping over to the other tank, and it hasn't happened yet!
Unless you are heating the camper with your on-board propane furnace, one 30 pound tank of gas will probably last you all summer. Once you start using your furnace for heat, you'll be running through a 30 pound tank about once a week or more.