An email from Banjercito this morning confirms this is just another rumor. It is quite simple to go to the source and get accurate information.
From the Official SAT page on Temporary Vehicle Imports regarding motorhomes:
"Validity
The term of the temporary importation of a mobile home is of 10 years with effect from the date of beginning of the “temporary importation permit of the mobile home”, with multiple entries and exits during its validity period."
The Official RulesThere is no mention of any deposit for MHs or trailers.