I'd suggest that if you're not wintering in the south already and leaving from there, that you leave Rhode Island in Feb/March, head south and then begin your trip cross-country from Florida or westward. Your big issues is leaving your area with possible winter weather to contend with.
Since you have unlimited time you might consider spending the winter in a snowbird location and then start your big trip from there. Then you won't have to worry about getting out of Rhode Island in bad weather.
Once you're south the weather is good for traveling from February on.
When you approach the mountains - and you'll want to see some of those areas, too - it won't be until May for safe travel in the mountains and then you'll still have to watch the weather forecasts as you get closer to the area you want to explore.
To begin your trip I'd suggest heading along the I-10 route or nearby secondary highways. We tend to get off those interstates to see some of our wonderful country rather than billboards. Sometimes you have no choice depending where you're heading. Don't be afraid of some mountain driving. It's the only way you can get to major national parks, etc.
Purchase the 'Mountain Directory for Truckers and RVers' It comes in both Western and Eastern editions. Even though we're in the mountains all the time we still refer to it to refresh our memories on some roads. There are many mountain roads that RVers can do very easily....and some you should stay off.
This book is cheaper from the publisher itself than from Amazon.
Once you get to California you can head north and then east again. The weather will be better at this time.
A good starter would be to get a road atlas (WalMart has nice cheap ones). Get a highlighter and go through the states you plan to drive and highlight things of interest to you. Then make your route from there.
Then come back to this site and ask about certain areas or states. It's hard for us to give suggestions for the whole U.S. if we don't know what states you'll be covering and what your interests are. Will you do some light hiking? Do you enjoy public parks (you can get the Senior National Pass which saves a lot of $$ on entrance fees and 1/2 off on campground) or RV parks? Big cities or more rural? Historic places or scenic places?
Have a great trip. Take your time and enjoy!