Your topic questioned the differences between a tarp and an RV cover. We answered that by saying a tarp is a bad idea and an RV cover is made for covering RVs so they are a better option.
As for whether you need one or not, a trailer is designed to be outdoors and for this doesn't truly need a cover. However, RVs don't get the love and attention like our vehicles. For most of us we wash and wax our cars but they will still age and so will an RV which is covered mostly by plastic and rubber. If you periodically check for holes or problems, this is the best thing to do.
A cover helps prolong the finish of an RV but it does not eliminate the care and maintenance. The covers keep dirt, debris, weather and bird droppings off the rig to prolong the RV finish but it still has drawbacks when compared to the best protection of a garage. But these do wear out. I've seen some covers so beaten up by the elements they are falling off the rig, So in themselves there's additional cost, care and maintenance.
A garage, while being the best solution for any vehicle storage has the disadvantages of cost to build, they take up a lot of room, they are in a fixed location and illegal in some Association neighborhoods. RV covers are a better choice when cost and portability are preferred. But, they do wear out because they are beaten up by the items and elements that are subjected while doing their job to keep the RV from these same items and elements.
Knowledge of advantages/disadvantages of all options is key. Then you can make the choice for yourself - and you do have the choice to change your mind. I chose not to cover mine and I don't perform enough care and maintenance either, but I've living with my choice just fine. But you don't want what option you pick to cause damage either.