Welcome aboard, Gary and Maggie! The 3685FL is a beautiful model, a very good choice. In my opinion, ANY of the Big Three's diesels will do you just fine. For point of reference, we do not have a 3685 but our 3295RK scaled (CAT scale back in May) at 20 pounds more than the GVWR for the 3685FL and we tow it with a diesel with no problem.
As for fulltimers joining 'clubs'. Some join campground membership groups that have dedicated campgrounds at which you can get a discount. Others, like us, join Good Sam, Passport America, Escapees, and maybe some others to get the discounts but those discounts are not really campground clubs. They're discounts at regular campgrounds that signed up to give the discounts in order to get the business. So we just look in their directories for rv parks that give discounts to members. Those are our first choice in rv parks. But, if none are in the area we want then we go to another rv park and pay the regular price. The Trailer Life directory shows not only parks that give discounts to Good Sam members but many, many other rv parks. Likewise for Woodall's Campground Directory.
We sold our stick home and pulled up stakes. We then became South Dakota residents (really easy to do) because in SD there is no state income tax, no property tax on vehicles, just a 3% excise tax on trade difference when you trade vehicles or rv's, and the 4th lowest insurance rates in the nation. We use Alternative Resources in Sioux Falls, SD, as our legal address and have them forward our mail weekly to where we tell them to send it. There's a similar service in Emery, SD, but I cannot remember its name. Also, Escapees in Texas provides mail service and you can become a Texas resident. Likewise for American Homebase (I think that's correct) in Pensacola, FL. Or you can just maintain your current residence and have any of those handle mail for you. There are other options I'm sure others will list for you.
Don't hesitate to ask questions. That's how the rest of us figured it out, too!
I hit the Submit button too soon. We sometimes make plans well in advance when we want to go somewhere that's very popular and will require reservations early to get a choice of RV Parks. We just spent a month in Hungry Horse, MT, next to Glacier National Park. That we booked a few months ago. Today we arrived in Island Park, ID, next to Yellowstone. That we booked awhile back, also. We also will attend the rally in Minden, NV, (near Reno/Tahoe/Carson City) in early October. But we have three weeks between here and there and have not yet decided where we will go. Some places you really need advance reservations. Many others you do not and can just decide today where you want to go tomorrow or next week or next month.
Fulltiming is something you will learn to adjust to fit your personal desires and needs. It will be what you make it. So just start out and adapt as you find out what you like and don't. Most of all, have fun! Next Sunday marks 29 months of fulltiming for us....and we're just getting started. If you want to get an idea of some places to visit, click on "Our RV Travels" in my signature block below.