Your motorhome, travel trailer, fifth-wheel or other RV may sit for weeks, maybe even months, exposed to elements including wind-driven dirt and dust, snow, bird dropping, tree sap, and acid rain. Especially harmful are the sun's ultraviolet (UV) rays that can cause paint and interior surfaces to fade, while vinyl, rubber, leather, and fabrics can dry, crack, and deteriorate. Unless you are the rare individual that has a permanent garage in which to park your RV, you will have to rely on protection from an RV cover or portable garage.
Unless you throw an old tarpaulin over your rig, not recommended, the least expensive protection comes with a RV cover. While there are different cover designs and materials available, the most important consideration is ease of use. Therefore, it is important to find one that is easy to handle and store. After all, a cover does no good if it is seldom used because it too hard to put on and take off. Unless you are covering a small conversion van or travel trailer, count on installing a cover being a two-person job. The better designs allows for installation without a step ladder, however.
Custom it covers made for a particular RV provide better protection than universal or ready fit covers because they follow the exact contours of the vehicle. For instance, custom-fit covers will not move around in the wind as much as an ill-fitting, universal-type cover. When ordering custom-fit cover make sure you include provisions for such things as roof-mounted air conditioners, mirrors, spare tires and other appendages. Covers will also usually have, either as standard or optional equipment, openings for doors so you can access the vehicle without removing the cover. Openings either use zippers or Velcro fasteners, with the latter having the advantage that they do not scratch or chip paint. They will also better work in winter when moisture freezes. While metal zippers are more durable than nylon ones, they are more likely to scratch finishes.
In reality, it is probably a toss-up as to which fabric is best with probably the biggest differences being the weight-per-foot of material, the length of the warranty that can range from two to five years, and of course, the cost. Virtually all covers are "breathable" to allow moisture to escape. If cover does not breathe well, corrosion-causing moisture can get trapped against the vehicle' finish and promote rust. Breathable covers help reduce the formation of mold and mildew, and in winter, ice. Breathable covers also allow heat to escape to keep interiors cooler. A good cover will also be water resistant and will provide protection against UV radiation.
Speaking of UV damage, it is important that tires are protected. Indeed, rather than wearing out, tire's biggest enemy is natural aging as well as ultraviolet or ozone damage. For vehicles used frequently, oils in the tires come to the surface during flexing and protect the rubber from ultraviolet light. Also when left idle, natural aging may cause the rubber to crack prematurely - especially in the sidewall area. Both UV and ozone damage can be greatly slowed when protected under an opaque waterproof covering so they are shielded from direct sunlight.
Most important is how the material is made into a cover. Good covers will feature seams that are overlapped, with double needle stitching for extra strength. Also there will be non-scratch, reinforced grommets located on each side of the cover for use with the tie-down rope. If available, soft interior liner will better protect RV surfaces, but can result in a heavier cover.
Next up in cost and convenience are portable garages, of which there are many styles available on the market. Typically, these use a steel or aluminum frame covered with a fabric material much like that used for RV covers. If you are considering a portable garage, first check to make sure that they are allowed by your local building codes and neighborhood covenants about "temporary" structures.
Big advantages of portable garages are that you can drive in and work in and around an RV with ease that is if you buy one that is big enough. It is important to check the construction of the frame and how the fabric is attached to this framework. The fabric used on portable garages, like covers, must provide protection against sunlight damage and keep the RV moisture free. Portable garages used in northern climates must be able to handle snow loads and in windy area have to be securely anchored and constructed so they do not take off like a kite. Again, when ordering make sure to account for appendages like air conditioners, antennas and so forth.