I work at Home Depot. I am a certified Paint Associate (LOTS of training) for what it's worth.
When it comes to paint, prep is everything and the proper tools are a close second.
First you need to clean the walls with TSP or TSP Substitute. Follow the directions on the package. Mix up in a bucket of water, clean the walls with a sponge, rinse the TSP off with clean water and a sponge (rinse the sponge out frequently and change the water as it gets dirty). Let the walls dry completely.
Use Kilz 2 latex primer on walls. I suggest two coats but you will most likely ignore that and only use one coat because you think you are saving money.
Buy the Behr Ultra (the one with the primer). The reason for the two primers is that the Kilz is helping the paint adhere to the walls. The primer in the Behr Ultra covers stains and lays down a better finish. The reason I suggest the Behr Ultra over the Behr Premium is because it is thicker and just plain covers better, especially if you get a dark colour (get your primer tinted at the paint desk) or use a translucent colour like yellow and red. If you don't paint much, you will get a better finish with Behr Ultra. You will need TWO COATS. THREE if you are using a red or yellow. Tinting the primer a bit for DARK colours makes the paint go a tad farther. On Ultra, the first coat is the "primer" and looks bad, second coat is the coverage coat. If using red or yellow, you may need a third coat as it may not cover completely on the second coat. It's the colour not the paint.
Don't be stingy with the paint. Too many people will not roll on enough paint in a single application trying to stretch the paint coverage. A gallon of paint covers about 250 to 400 SF (I generally figure a gallon at 300 and do not factor in windows and doors). Multiply the length of your walls X wall height to get SF. Go to the Behr How to website BEFORE you paint
http://www.behr.com/consumer/how-to/interior for info on HOW to paint.
Use the foam rollers and buy several (like a pk of 6) of both 4" and 6". You can save a bit on the rollers by slipping them in a plastic bag or bit of plastic wrap of aluminum foil, wrap airtight, then toss in the fridge for a few hours to overnight or the freezer for longer (I've done the freezer for up to two days). Thaw completely before unwrapping and using again. Good for when you need to break for lunch or overnight.
The small roller pans are good for painting inside a manufactured RV (small and compact) as they fit on the counters, etc better (been there/done that with the Class C... even wallpapered the ceiling with pressed tin tile look anaglypta wallpaper).
Both Wooster and Purdy make a really short handled paint brush that is good for cutting in paint in tight spaces. Use a good paint brush such as the Wooster or Purdy or the ones listed as "BEST" on them. These are wash and keep brushes. Clean them out well after using.
PAINT SHEENS:
For an RV, Don't use "Flat" or "Eggshell". Use either "Satin" or "Semi-Gloss" on the walls, "Semi-Gloss" on the trim. The shinier the finish the easier it is to clean. I suggest you stick with a semigloss for everything.
I used a Semi-gloss (custom match pale yellow) on our partition walls and on the cabinets (Behr Embellished Blue 510D-4) both of which were unfinished wood (we used flush hollow core luan doors for our walls that are at a 90 degree angle to our long walls). Our long walls (the long sides of the bus) are covered in Ashely Oak click lock laminate floor that we used as wall paneling. The PVC window trim will be primed in a custom tinted med grey Kilz 2 then 2 coats of Martha Stewart Living "Silver Leaf" Metallic paint (brushed on)... just like the pressed tin look anaglypta wall paper on the ceiling (when I get that).
For outside, I will be repainting the rub rails and other "trim" bits with the Marquee exterior to see how well that sheds the dirt. If it does well, we will prime then roll the whole bus with a new coat of paint after we get to TX and on our own place.
DO NOT BUY SPEED-WALL or AMERICA's FINEST AS YOU ARE JUST THROWING YOUR MONEY AWAY.
I know nothing about Lowes paint except I didn't use the junk prior to 2011 before I started working at Home Depot. I used ACE brand when it was made by Pratt-Lambert. I know nothing about the new bunch. Prior to working at Home Depot, we remodeled houses for resell and I was the decor/paint "consultant".