(petitioners, married couple, resided in Quincy, IL, but traveled for business; I.R.C. Sec. 163 allows deduction for mortgage interest on principal residence and one other residence as elected by taxpayer; neither Code nor regulations detail how election to be made or timing of election; petitioners had motor home on which they claimed mortgage interest deduction; court agreed with IRS and all deductions other than for interest expense should be disallowed under I.R.C. Sec. 280A(a) due to lack of substantiation the identifiable portion of motor home used for business; while IRS asserted that petitioners couldn’t claim I.R.C. Sec. 162 travel-related expenses because petitioners lived in motor home and were, as a result never away from home, some expenses allowed during exam and court dismisses argument of IRS on this point).