Slovenia is a stunning country, manicured and spotless..it makes Switzerland look grubby! Croatia is equally stunning but less sophisticated. We've done both several times without a hitch in our 34 footer.
As with any trundle, take your time and enjoy them both, and you shouldn't have any problems.
We did a 57 night tour last year covering France, Belgium, Luxembourg, Switzerland, Italy, Slovenia, & Croatia. We only did a small part in Slovenia and it does have mountainous hills, as do Croatia and Switzerland and Italy. Non-motorway roads were quite narrow but in good condition (unlike UK) but we managed fine with a 29ft B-class.