How long your propane tanks will last depends on too many variables for a simple answer. Just like in your home, it depends on your usage . . . and the weather.
After a year-and-a-half of full-timing, I'm just now nailing down the perfect balance between propane use and electrical use. If I understand your post, you'll have full hook-ups for a 3 day outing. I'll assume that means your electricity is included in the basic price of the campsite. In that case, I'd use electric for refer and water heater; and if the weather doesn't get below freezing, I'd use a space heater/electric fireplace for my primary heat source. I run an extension cord from the heater, out my slide gasket, and plug it directly into the 15 amp outlet on the CG pedestal. That way I don't pop a circuit breaker when the heater, refer, water heater, water pump, television, and coffee maker are all running at the same time. That means your propane use will be limited to refrigerator during transit, and cooking. If your trip is short, and/or you use the campfire or charcoal for cooking, you could precool the refer on electric (before you leave home), then turn it off until you plug into CG electric, and use virtually no propane at all.
On the other hand, I'm currently staying at a full-hookup site for 4 months, and my electricity is metered separately from my campsite fee. In December, I heated mainly with my electric fireplace during the day, and only used my furnace at night to keep my water lines from freezing (overnight temps have frequently gotten down to or below 30*). I only burned through 1 propane tank, but my electrical charge was $92.00! Since propane right now is dirt cheap (around $18/30# here), I decided to use my furnace more for heating this month and turn my WH on only a few minutes before I need it (for a shower, washing dishes, etc.), then turning it off when I'm done. Even using both my oven and cook-top quite a bit, so far I've only burned through one tank of propane. We'll see if I save any money operating this way.
One other thing; if you're using the built-in propane quick connect for your grill, go easy on the pre-heating and post-cooking burn-off. Your grill will go through a lot of propane very quickly
This is my personal experience. Your mileage may vary depending on your habits. But from the sound of it (using your RV only a couple of weekends a month), your two 30# bottles should get you through most/all of the season.
Hope this helps! Happy Camping in your new rig!