I have little to offer in terms of advice, but will be following this thread with interest since we are in the market for a trailer of roughly the same size, though we may need to go up to about 24 feet before it's all said and done. We did some hard shopping yesterday, seems that at our size point most of the fresh tanks are around 27 gal, with a few models having 40 gallons.
Most of our camping is sort of a midway between true boondocking and campground facilities. We tend to camp at events where sometimes there is power (for a charge), but almost never any water or sewer hookups. We typically only camp for a weekend, 3-4 days max. When we had the old Bounder, it had a 60 gal fresh water tank, but for most trips I only filled it half full (OK, mostly because I was afraid the tank might fall off onto the highway, but point stands: two adults got on just fine on 30 gallons of water for a long weekend)
Someone posted a web guide to dump stations earlier, use that to help plan. Just about any private campground you would pass on your route will let you dump for a small fee. Another source we have used a lot are some of the bigger RV dealerships have dump stations. Some free, some a small charge.
One holding tank tip is to divert gray water from showers, dish-washing, and such into the black tank. You typically don't use anywhere near as much capacity in the black, and the extra water helps at dump time. We keep a small bucket in the shower to catch as much shower run-off as possible, especially those first couple of minutes when the water is getting up to temperature. Then the bucket is poured into the toilet. After washing dishes, the dishpan is also emptied into the toilet. Saves space in the gray tank, and puts soapy water in the black.