Okay, a bunch of thoughts....
1. The Colonial Beach area is nice but very rural. It's not near anything (rail or metro or major transportation artery). And it's a long way from DC.
2. If you're in Manassas, you actually have a very convenient way to get in to DC. Get thee to VRE (Virginia Rail Express):
Rail service in Virginia - VRE VRE if fast, efficient, comfortable and basically is a commuter service (so runs in to DC in the morning, runs out in the evening and don't plan on staying too late). There are a couple of stops in Manassas (the prime one is in old town Manassas at the train station which is a nice little place to photograph). Seriously, it's worth driving 30 minutes (if you're on the outskirts of Manassas) to take VRE rather than attempt to commute in. If I was driving from Manassas (and I grew up there), to avoid rush hour traffic I'd leave at 10am and plan on hitting the road back to Manassas at 2pm and assume it would take me about an hour each way. That means you'd be in DC 11am-1pm. With VRE you can sleep, read the paper, prep your camera gear, work on your laptop, have cell coverage, and do it all while sitting down. Besides, you'll enjoy the break from being in the RV.
3. Staying overnight. Unless you have friends, be prepared for sticker shock. This ain't NYC. But even low-rent chains (La Quinta, etc.) will cost you 3 figures. Either just decide to get something special (and rent at the right location or a special B&B or one-of-a-kind hotel). Or stay at a hotel in Virginia on the orange line (for instance, the Hyatt in Rosslyn...where you can walk to the Iwo Jima Memorial to shoot the sunrise and then in 5 minutes be at Key Bridge for more sunrise shots and you're near the Rosslyn Metro stop to get in to DC). Check out the DC Meetup thread for some of the sunrise Iwo Jima Memorial pictures.
4. Recommendations on what to see/do.
--You're planning on Monticello. Definitely get the tour of the garden/orchard (even though most of it is harvested at this point). The leaves are now changing in Virginia and given the diversity of Mr. Jefferson's trees you should have a lovely mix of foliage on the mountain. You will not be allowed to take pictures inside Monticello itself. If you're interested in Presidential houses, you've also got Mt. Vernon locally (well worth a trip to), and Madison's place near Monticello (Montpelier) which is less crowded than Monticello.
--Not on your list but definitely worth a short drive is to hit Harpers Ferry (old town, 3 rivers converge, lovely waterfall that is easily accessible, in the mountains, good foliage) which is close to the Old Bavarian Inn (outstanding view to the Potomac, great German food) and then extremely close to the battlefield of Antietam. I've walked through probably 3 dozen Civil War battlefields. Antietam is the best for it's combination of: being relatively unchanged from the actual battlefield (rather than developed and broken up or commercialized), easy to understand (1 day battle, fighting started in the north, moved to the center, the day ended in the southern part of the field so there's an easy progression), historically momentous (bloodiest day in American history, made the Emancipation Proclamation possible), tremendous drama and pathos, and it's not a hassle to get to or trek through...you can do Harpers Ferry, Old Bavarian Inn, and Antietam in a day or less from DC in an RV.
--Provide more details about what it is you're interested in seeing/shooting and a bunch of us locals can offer tips. A great place for a sunset in DC is the Jefferson Memorial (you can catch the sun setting and the Potomac as well from inside the Memorial). Too bad you're leaving on the 21st--I'm coordinating a photo meet up and night shoot of the US Institute for Peace (in DC) the night of the 21st. It's also near the Vietnam Veteran's Memorial, the Einstein Memorial, and the Lincoln Memorial...all great locations for shooting at night. If you were in town you'd be welcome to join.
--RV parking in DC will be tough.
