The main mistake is expecting to get your money back out of the trailer later. You won't. You get your money out of these improvements by using the trailer and enjoyment.
Second major mistake IMHO is removing a system (like removing all propane), or removing/eliminating an appliance (removing
furnace for example).
A third mistake is making it really personal. I've seen some trailers that are a full on Coca-Cola theme interior. Fine if you are into Coke, but it eliminates most future buyers.
On antiques, I slowly learned I had made the same mistake as you describe. Original finish and patina is worth a lot more than a stripped and refinished antique, even if the finish is in mediocre condition. Original finish and patina can represent 75% or more of the value.
I've kept the original cushions (replaced the foam) and curtains in my Trillium for a couple reasons. First, they were still in good condition. Given a choice between keeping original in good condition and replacement, I'll keep it original. Second was cost. Keeping original cushions and curtains was a lot cheaper and took a lot less time than replacing them.
I don't paint unless it is needed for maintenance. So
painting the outside or the inside cabinets or ensolite? Nope. Original as long as it is good condition will require ZERO maintenance basically forever. I did have to replace several rotten cabinet doors. So I went ahead and painted all the cabinet doors, replacing several of them.