The biggest costs associated with most multi-stop road trips are: lodging, fuel, attractions, food, and misc. Here's how to cut some of them down to size.
The More the Merrier: Most big costs (lodging and fuel) can be cut in half by traveling with friends. A van that gets 20 mpg might not sound very good but when split between 4 friends that's more like getting 80 mpg each. If you let him/her drive while you sleep you might even get there faster, save money on lodging, and spend more time on the fun stuff.
"But what if I want to do my own thing?" I hear you say. Well, you or your buddy(s) could drop the other off and come back later, you could rent a car if you really split up, or you could bring your motorcycle (or scooter, or motorized bicycle, or paramotor, or pack mule, or....well, whatever you want to use) in the back of a truck, on a hitch carrier, or on a trailer...of course, renting a car is also an option.
Try Camping: Two weeks on the road staying in $100 per night hotels would cost $1400 whereas camping in priceier national-chain campgrounds like KOA (aproximate costs for a tent site: low - $14, average - $28, high - $50) cuts this down to $392 ...and you are much more likely to meet people, do something fun, and remember the experience. They often have some entertaining ammenities such as: gamerooms, a camp kitchen, pools, kids activities, fishing, movie night, etc. Better still, staying in state parks or national forests gives you beautiful surroundings for even less. Some national forest campgrounds are even free. A great app for finding campsites and related services is Allstays Camp and RV. So, what if you don't have camping gear? - A decent tent, sleeping bag, and air mattress will pay for itself in less than a week - you can probably even borrow them from an outdoorsy friend. How about 2 weeks lodging costing $0? try boondocking, which is camping for free in undesignated sites. By far, this is my favorite technique because it allows me to travel on my own terms without worrying about making reservations or if there will be rooms available...and its easier than you think. To do this you really need to be able to sleep in your vehicle. If you have a station wagon, SUV, van, or truck/camper this should be easy. A great app for finding spots is the appropriately named BoonDocking.
Cook on the Road: Eating out full time can really kill your budget. $9 meals x 3 meals per day x 14 days = $378 per person so if you can cut this in half you can easily pay for the gear needed. The best part of this is that you don't have to choose between hot take-out meals and cold sandwiches...and the time you spend preparing and consuming meals will be much more memorable than the umpteenth time you have eaten at Taco Bell. Perhaps the best source of information and gear selection may be Appalachian Trail hikers (or websites dedicated to the topic). These people carry their entire kitchen with them and very rarely have opportunities to eat out. Their gear tends to be very light and compact...and sometimes surprisingly cheap (how about a stove for less than $1? My preferred kitchen cost me less than $10 and weighs less than 2 pounds including fuel.) Some of them even manage to cook fairly elaborate meals (one guy I ran into on the AT had been experimenting during his entire hike and was planning on trying deep frying next). Don't worry, you will still be able to eat out when you want but now you can afford nicer restaurants while still lowering your food budget.
Cut down on Bulk: The less you bring with you the better your experience...I promise! No one wants to have to unload the entire car to get to that one thing that is buried under a trunkload of suitcases...things like this just make the trip much more of a chore. The less stuff you carry the less hassle you will have, the more friends you can travel with, the better your gas mileage (especially if you can take a smaller vehicle), the easier it will be to camp/boondock, split up and explore, and the more you will focus on the experience instead of a mountain of stuff you are dragging along. After all, isn't the point of traveling to get away from your everyday life and have new experiences? See also: Cocooning (trend)
Keys to cutting down on stuff:
#1 - Do laundry (imagine packing 14 outfits for a 2 week trip vs packing 3 or 4 days worth of clothes) Try to pick a selection of clothes that all match and are versatile...the same strategy applies to shoes and jackets etc. (I wear khaki shorts/pants and blue polo shirts so everything matches and looks acceptable anywhere from the hiking trail to a moderately nice restaurant or live musical). If you have a strategy with clothes it saves room because you won't need extra matching shoes, socks, belts, and jackets to go with an outfit. Laundry facilities can be found at virtually every campground, truck stop, hotel, national park, and of course every city has one. Going to a cold and warm climates on the same trip? - Bring thermal underwear - it doesn't take up much space, matches anything because it is not visible, and converts your warm weather clothes into cold weather clothes.
#2 - Don't be a Sissy Seriously, if you are terrified of experiencing any inconvenience, discomfort, or surprises you aren't going to have much of an adventure on your trip and might as well stay home. This means only taking what you know you really need/will use. Don't forget that if you really need something that you didn't bring you are probably going to be somewhere near people...some of them probably have the same need...some place nearby will sell it.
#3 Bring Travel Sizes Only No travel sizes of your favorite shampoo, pain reliever, lotion, or laundry detergent? - buy small bottles and make your own. Make sure to keep these in a dedicated travel bag year-round - eventually you will have everything you need in 1/4 the space and it will be ready to go next time. This also applies to electronics (my Iphone instead of my computer) and books (E-reader, phone, audiobooks).
#4 Remember the Purpose of your Trip - If you bring work (of whatever sort) with you it probably won't get done and you will just end up lugging it around. Bring a (small) hobby or two if you want but don't go overboard.
I hope you find at least a few of these suggestions helpful. Happy Travels!