As far as your "is it a good idea" question, Yes, you will be fine. We met a girl who was travelling solo and camped in a tent with no issues. You are right on the beach and surrounded by other campers as well. Me and my wife camped in a tent at Ivans as well for 3 nights once and were fine. It was "roughing" it and if you have a cabin it will probably be more convenient than the tent. As someone said many of the expensive places do not even have locks or give you a key for the room. Just out of habit/comfort, I carry my wallet and money with me or if I have a renatal car, lock it in there.
If you don't bring anything...bring toilet paper as at times the restroom at Ivans may be out and is often restocked by the campers. Just don't want you to be "stuck" when it may be needed.
We always go "in season" and can pick up groceries at the market or there is a little store that my pal Tony runs close to Foxy's that has some food and plenty of beverages. I can't provide much info on the stores in the off season though.
Google the Ferry Schedule for St John to Jost Van Dyke but the ferries are usually reliable and stay on schedule. it looks like it Lvs St John Fri Sat & Sun at 8am or 2pm and on M, T, W at 8:30am and 3:30pm it Leaves Jost on Fri Sat & Sun at 9:15am or 3pm and on M, T, W at 9:15am and 4pm.
Only day you may have an issue is Thursday...BUT you can always look to ferry to West End Tortola from St John or From Jost as that runs more frequently and then they offer Ferries from West End to the Us VI. Keep in mind that if your last day you are leaving Jost to catch a flight home from St Thomas to allow about 3 hrs for travel to get to airport as the ferry goes to St. John, you get off for Customs then reboard then leave for STT but that ferry goes to Red Hook, which is then about a 40 minute cab ride to the airport. This is where we have found it easier to get the return ferry from Jost to West End Tortola, and then do a separate ferry from West End to STT (Charlotte Amalie). This is then a 10 mninute cab ride to the airport once back on STT.
No problemwith luggage on the ferries (they charge a few $$ per bag) you get on and off at a dock. You may have heard about some of the charters or daytime water taxis that come up on the beach where luggage could be difficult as you are getting out on the sand or maybe in the water.
One tip, especially in the "off season" is plan ahead or check ahead for your meals if you are looking to eat out, especially dinner. It is not always like a walk in and sit down and order, as alot of places want to know you are coming ahead of time and sometimes what your food selection is. Highly recomend One Love for any food. Best food on the island and you can walk there from Ivan's. If you are at Ivans on a Thursday evening do his BBQ.(if offered that time of season)
Have fun...Best place on Earth. If you go to One Love, tell Seddy hello and he may do some magic tricks for you. Say his friend Al (that does magic with him) told you to go there and says hello!